CORNELL UNIVERSITY. THE Roswell P. Flower Library THE GIFT OF ROSWELL P. FLOWER FOR THE USE OF THE N. Y. STATE VETERINARY COLLEGE 1897 2757 Digitized by Microsoft® | | Cornell University Library | QP 2 pt Digitized by Microsoft® Studies in general physiolog wi This book was digitized by Microsoft Corporation in cooperation with Cornell University Libraries, 2007. You may use and print this copy in limited quantity for your personal purposes, but may not distribute or provide access fo it (or modified or partial versions of if) for revenue-generating or other commercial purposes. Digitized by Microsoft® THE DECENNIAL PUBLICATIONS OF THE UNIVERSITY OF CHICAGO Digitized by Microsoft® THE DECENNIAL PUBLICATIONS ISSUED IN COMMEMORATION OF THE COMPLETION OF THE FIRST TEN YEARS OF THE UNIVERSITY’S EXISTENCE AUTHORIZED BY THE BOARD OF TRUSTEES ON THE RECOMMENDATION OF THE PRESIDENT AND SENATE EDITED BY A COMMITTEE APPOINTED BY THE SENATE EDWARD CAPPS STARR WILLARD CUTTING ROLLIN D. SALISBURY JAMES ROWLAND ANGELL WILLIAM I. THOMAS SHAILER MATHEWS CARL DARLING BUCK FREDERIC IVES CARPENTER OSKAR BOLZA JULIUS STIEGLITZ JACQUES LOEB Digitized by Microsoft® THESE VOLUMES ARE DEDICATED TO THE MEN AND WOMEN OF OUR TIME AND COUNTRY WHO BY WISE AND GENEROUS GIVING HAVE ENCOURAGED THE SEARCH AFTER TRUTH IN ALL DEPARTMENTS OF KNOWLEDGE Digitized by Microsoft® Digitized by Microsoft® STUDIES IN GENERAL PHYSIOLOGY Digitized by Microsoft® Digitized by Microsoft® STUDIES IN GENERAL PHYSIOLOGY BY JACQUES LOEB = FORMERLY OF THE DEPARTMENT OF PHYSIOLOGY NOW PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CALIFORNIA THE DECENNIAL PUBLICATIONS SECOND SERIES VOLUME XV PART I CHICAGO THE UNIVERSITY OF CHICAGO PRESS 1905 + Digitized by Microsoft® jo 2, vif eo > oa gil is ‘ No. 4646 Copyright 1905 BY THE UNIVERSITY OF CHICAGO Digitized by Microsoft® ) PREFACE I sHoutp not have had the courage to offer these volumes to the public, had not requests repeatedly come to me from physicians and biologists to render my publications, which are widely scattered, more easily accessible. There- fore, when the editor of the “‘Decennial Publications of the University of Chicago” invited me to make a contribution to the series, I mentioned to him, not without hesitation, the idea of collecting and republishing my papers on General Physiology. Through his initiative and kind as- sistance the idea has been carried out. No one will expect that a collection of papers on very diverse subjects can form attractive reading matter. Yet I may mention, by way of an apology, that, in spite of the diversity of topics, a single leading idea permeates all the papers of this collection, namely, that it is possible to get the life-phenomena under our control, and that such a control and nothing else is the aim of biology. Thus the reader will notice that in a series of these publications I have tried to find the agencies which determine unequivocally the direction of the motion of animals, and he will also notice that I consider a complete knowledge and control of these agencies the biological solution of the metaphysical problem of animal instinct and will. In taking up the problem of regeneration I started out with the idea of controlling these phenomena, and considered it my first aim to find means by which one organ could at desire be caused to grow in the place of another organ. Thus the experiments on heteromorphosis originated. As far as the problem of fertilization is con- cerned, it seemed to me that the first step toward its solution should consist in the attempt to produce larve artificially from unfertilized eggs in various classes of animals. ix Digitized by Microsoft® x PREFACE It seemed desirable that the reader should be spared an undue amount of repetition, and for this reason a number of publications are omitted from this collection, and those printed are in many cases shortened. Among the papers which have been omitted are the preliminary notices and all those papers of which I am not the sole author. Occasion- ally I have made additions in the form of footnotes. Such footnotes have always been marked by the addition of [1903] at the end. Only a small number of these papers appeared originally in English, namely, VII, XXI, XXVI-XXXV, and XXXVII. The other papers were translated from the German by Pro- fessor Martin H. Fischer, to whom I wish to express my sincere thanks. The credit as well as the responsibility for the translation belongs entirely to him. In the reading of the proof I was assisted by Dr. Fischer, Dr. Rogers, Dr. Bullot, and Dr. Bancroft. Mr. Rogers made the index for the first volume. To all these gentlemen my thanks are due. Jacques LoEs. BERKELEY, CALIFORNIA, October 14, 1904. Digitized by Microsoft® II. IIT. IV. VI. VII. VIIL. IX. XI. XII. XIII. XIV. XV. TABLE OF CONTENTS PART I The Heliotropism of Animals and its Identity with the Heliotropism of Plants Further Investigations on the Heliotropism of Ani- mals and its Identity with the Heliotropism of Plants On Instinct and Will in Animals Heteromorphosis Geotropism in Animals Organization and Growth - Experiments on Cleavage - The Artificial Transformation of Positively Helio- tropic Animals into Negatively Heliotropic and vice versa On the Development of Fish Embryos with Sup- pressed Circulation On a Simple Method of Producing from One Egg Two or More Embryos Which Are Grown Together On the Relative Sensitiveness of Fish Embryos in Various Stages of Development to Lack of Oxygen and Loss of Water On the Limits of Divisibility of Living Matter Remarks on Regeneration Contributions to the Brain Physiology of Worms The Physiological Effects of Lack of Oxygen x1 Digitized by Microsoft® 89 107 115 176 191 253 265 295 303 309 321 338 345 370 TABLE OF CONTENTS xu PART II XVI. The Influence of Light on the Development of Organs in Animals 425 XVII. Has the Central Nervous System Any Influence upon the Metamorphosis of Larvee? 436 XVIII. On the Theory of Galvanotropism 440 XIX. The Physiological Effects of Ions. I 450 XX. On the Physiological Effects of Electrical Waves 482 XXI. The Physiological Problems of Today 497 XXII. The Physiological Effects of Ions. IT- 501 XXIII. Why Is Regeneration of Protoplasmic Fragments without a Nucleus Difficult or Impossible ? 505 XXIV. On the Similarity between the Absorption of Water by Muscles and by Soaps 510 XXV. On Ions Which Are Capable: of Calling Forth Rhythmical Contractions in Skeletal Muscle 518 XXVI. On the Nature of the Process of Fertilization and the Artificial Production of Normal Larvee (Plutei) from the Unfertilized Eggs of the Sea- Urchin 539 XXVII. On Jon-Proteid Compounds and Their Role in the Mechanics of Life-Phenomena.—The Poison- ous Character of a Pure NaCl Solution 544 XXVIII. On the Different Effects of Ions upon Myogenic and Neurogenic Rhythmical ‘Contractions and upon Embryonic and Muscular Tissue 5b9 XXIX. On the Artificial Production of Normal Larve from the Unfertilized Eggs of the Sea-Urchin (Arbacia) 576 XXX. On Artificial Parthenogenesis in Sea-Urchins 624 XXXI. Onthe Transformation and Regeneration of Organs 627 XXXII. Further Experiments on Artificial Parthenogenesis and the Nature of the Process of Fertilization 638 Digitized by Microsoft® TABLE OF CONTENTS xii XXXII. Experiments on Artificial Parthenogenesis in Annelids (Cheetopterus) and the Nature of the Process of Fertilization XXXIV. On an Apparently New Form of Muscular Trrita- XXXV. XXXVI. XXXVIT. XXXVIII. INDEX bility (Contact-Irritability?) Produced by Solu- tions of Salts (Preferably Sodium Salts) Whose Anions Are Liable to Form Insoluble Calcium Compounds The Toxic and the Antitoxice Effects of Ions as a Function of Their Valency and Possibly Their Electrical Charge Maturation, Natural Death, and the Prolongation of the Life of Unfertilized Starfish Eggs (Asterias Forbesii) and Their Significance for the Theory of Fertilization On the Production and Suppression of Muscular Twitchings and Hypersensitiveness of the Skin by Electrolytes On the Methods and Sources of Error in the Experiments on Artificial Parthenogenesis Digitized by Microsoft® 646 692 708 728 748 766 773 Digitized by Microsoft® PART I Digitized by Microsoft® Digitized by Microsoft® I THE HELIOTROPISM OF ANIMALS AND ITS IDENTITY WITH THE HELIOTROPISM OF PLANTS! I. INTRODUCTION I intend to show in the following pages that animal movements depend upon light in the same way as the move- ments of plants. It is a well-known fact that animals, when light falls on them, move toward the source of light, like the moth, or move away from it, like the earthworm. It is also well known that certain plant organs have a tendency to turn toward or from the source of light when illuminated from one side only. While the conditions which govern the behavior of plants toward light have been well analyzed, especially by Sachs, little has been done to investigate the conditions upon which depend the movements of animals toward a source of light. It is the purpose of this paper to fill this gap, and to enumerate the facts which show that in reality the animal motions called forth by light depend upon the same circumstances as the motions which light produces in plants. The effects of light which we intend to study are purely mechanical, inasmuch as they consist in changes in position, as well as in the direction and the sense of the progressive movements of living animals. Consequently we shall regard as essential such circumstances as can help to explain the mechanical effects of the light. These circumstances, as in the case of all stimulations, are of a double origin: first, those belonging to the stimulus—in this case the light; and, 1 Pamphlet, Wurzburg, 1889. 1 Digitized by Microsoft® 2 STUDIES IN GENERAL PHYSIOLOGY second, those belonging to the structure of the organism. So far as the light is concerned, the circumstance which controls the orientation of the animal and the direction of its movements is the direction of the rays falling upon the animal.’ The condition which is of importance on the part of the animal is the symmetrical shape of the body. Sachs discovered that all plant organs which have a radial structure are orthotropic (this means that they bend, when light strikes them on one side, until their longitudinal axes lie in the direction of the rays of light), but that all dorsiventral structures are plagiotropic, 7. e., they place their surfaces perpendicular to the rays of light. Symmetrically situated points at the surface possess a quantitatively and qualitatively equal irritability. In this way the organ of a plant is mechanically forced to orient itself in such a way that the rays of light strike symmetrical points at equal angles to the surface. If the plant, as for example the swarm spore of alge, is capable of a progressive motion, it must of course, in order to maintain this position, move in the direction of the rays of light. This is, indeed, found to be the case. I shall now show that quite generally in animals the direction of the rays of light controls also the direction of those movements which are caused by light; that, in addi- tion, quite generally in animals their orientation depends 1 In these experiments it is presumed that the animals move under the influence of only one source of light. It is explicitly stated in this and the following papers that if there are several sources of light of unequal intensity, the light with the strongest intensity determines the orientation and direction of motion of the animal. Other possible complications are covered by the unequivocal statement, made and emphasized in this and the following papers on the same subject, that the main feature in all phenomena of heliotropism is the fact that symmetrical points of the photosensitive surface of the animal must be struck by the rays of light at the same angle. It is in full harmony with this fact that if two sources of light of equal intensity and distance act simultaneously upon a heliotropie animal, the animal puts its median plane at right angles to the line connecting the two sources of light. This fact was not only known to me, but had been demonstrated by me on the larve of flies as early as 1887, in Wiirzburg, and often enough since. These facts seem to have escaped several of my critics. [1903] Digitized by Microsoft® HELIOTROPISM OF ANIMALS 3 on the form of the body in so far as dorsiventral animals move with their median planes in the direction of the rays of light, in which position the rays fall upon symmetrically situated points of the surface of their bodies at nearly equal angles. In this way the fact that a moth flies into a flame turns out to be the same mechanical process as that by which the axis of the stem of a plant puts itself in the direction of the rays of light. In both cases, however—in the fatal flight of the moth as well as in the orientation of plants— one point remains unexplained, namely: how can the light so change the state of the protoplasm as to bring about the mechanical effects just mentioned? At present we are not able to form a clear idea of this. A second condition which has a determining influence upon the mechanical effects of light on plants is the refran- gibility of the rays. Sachs has shown that it is chiefly the more refrangible rays which are able to bring about move- ments in plant organisms. We shall see that quite gen- erally the more refrangible rays are also more effective mechanically in the animal kingdom. Thirdly, we shall prove that the orientation of animals as well as of plants takes place when the intensity of the light remains constant. Very often we observe, for example in our eyes, that a change in the intensity of the light acts as a stimulus. In addition to these essential considerations of the effects of light in the animal kingdom, the following factors play a réle, namely: Fourthly, light causes the orientation of animals (as well as of plants) only within certain limits of intensity. Fifthly, temperature influences the movements of orientation in animals and plants toward light—which is true for all phenomena of stimulation. To sum up: The conditions which control the movements of animals toward light are identical, point for point, Digitized by Microsoft® 4 STUDIES IN GENERAL PHYSIOLOGY with those which have been shown to be of paramount influence in plants. Aside from the problem of proving by suitable experi- ments the stated propositions, it is also necessary for us to show what réle the orientation toward the light plays in the economy of life of an animal. Ishall therefore first describe the experimental proofs of the identity of animal heliot- ropism with plant heliotropism, and then snow by individual examples what role heliotropism plays in the economy of life of animals. To discuss the latter point it will be necessary also to describe briefly the other forms of irritability pos- sessed by an animal. In a short article which appeared in January, 1888, I described the principal laws upon which depends the orien- tation of animals to light, and the identity of these laws with those governing plant heliotropism.* II. THE ESSENTIAL PHENOMENA AND LAWS OF HELIOTROPISM IN PLANTS Assuming that the reader is acquainted with the orienta- tion of plants toward a source of light, it will suffice at this place to call attention briefly to the essential facts which bear upon our subject. In so doing I shall follow the presenta- tion given by J. von Sachs in his lectures on plant physi- ology.’ Straight stems or roots of growing plants bend when light falls on them on one side only, or with greater intensity on one side than on the other, until their tips lie in the direc- tion of the rays of light. Those organs which turn toward the source of light are called positively heliotropic; those which turn from the light, negatively heliotropic. 1“ Die Orientierung der Thiere gegen das Licht (thicrischer Heliotropismus),” Sitzungsberichte der Wiirzburger physikalisch-medicinischen Gesellschaft, January, loss. 2 Vorlesungen tiber Pflanzen-Physiologic, 2d ed. (Leipzig, 1887). Digitized by Microsoft® HELIOTROPISM OF ANIMALS 5 It was formerly believed that the bending of the positively heliotropic parts of plants was due to the fact that the side which was turned away from the light grew more rapidly, because plants when brought into the dark at first grow more rapidly than they do in the light. But it was proved in Sachs’s laboratory that negatively heliotropic organs also grow more rapidly in the dark. Because of the similarity of the geotropic and heliotropic movement in plants, Sachs came to the conclusion that the direction in which the rays of light penetrate the plant tissue determines the orientation of the plant toward light. He also proved that not all the rays of the visible sun spectrum bring about heliotropic movements, but only, or at least chiefly, the more refrangible rays. The less refrangible rays, which are of importance in assimilation, are ineffective heliotropically. If the light be previously passed through a dark-blue ammoniacal solution of copper, which absorbs all the red, yellow, and a part of the green rays, the heliotropic bending occurs in the same way as in completely white light. If, however, the light passes through a saturated solution of potassium bichromate, which lets through only red, yellow, and a part of the green rays, “the heliotropic shoots remain straight and vertical, no matter how intense the light is which passes through the solution.” Finally, if the light “is passed through a solu- tion of quinine sulphate, the fluorescence of which completely absorbs the ultra-violet rays, the heliotropic curvatures nevertheless appear—a proof that they are caused princi- pally by the visible blue and violet rays.” The best proof of the theory that the direction of the rays of light controls the orientation of plants was found by studying freely moving plant organs, the swarm-spores of alge. These swarm-spores make progressive movements like animals, and Strasburger’ proved that they move in the 1 STRASBURGER, Wirkung des Lichtes und der Wdrme auf Schwiirmsporen (Jena, 1878). Digitized by Microsoft® 6 STUDIES IN GENERAL PHYSIOLOGY direction of the rays, to or from the source of light. The more refrangible rays alone exercise this effect on the swarm- spores. They behave in the light which has passed through an ammoniacal solution of copper just as in diffuse daylight. On the other hand, they are not affected by light which has passed through a potassium bichromate solution, by light from a sodium flame, or by the light coming through ruby glass. The chlorophyll-bearing protoplasm of cells moves under the influence of light.’ The chloroplasts of a thread alga, Mesocarpus, turn ‘their broad surfaces toward the sky so that the rays fall upon them at right angles. If the direction of the rays is changed, the chloroplasts turn so that their broad surfaces are again at right angles to the rays. Direct sun- light, however, causes the chloroplasts to assume another position— they place their surfaces parallel to the rays which strike them.” According to modern plant physiology, the whole proto- plasm of a multicellular plant is to be conceived of as a continuous mass, as a single protoplasmic body.? More recent investigations have shown that when a plant organ is illuminated, that side of the organ which becomes concave from the effect of the light becomes rich in protoplasm, while the opposite convex side becomes poor.’ Multicellular organs behave in this regard like unicellular ones. Thus it appears that the light forces the protoplasmic mass to move in such a way that positively heliotropic protoplasm wanders to the side of the organ which is turned toward the light, while negatively heliotropic protoplasm wanders to the opposite side.’ Should it turn out that this phenomenon really oceurs 1 STAHL, Botanische Zeitung, 1880. 2 Sacus, loc. cit., p. 94, 3 Wortmann expressed his observations in this way. It is possible that in reality protoplasm on the concave side is only more opaque than on the opposite side. This difference in optical appearance may simply be the expression of a difference in the size of the colloidal particles. [1903] +See WorTMANN, Botanische Zeitung, 1887. Digitized by Microsoft® HELIOTROPISM OF ANIMALS 7 in all cases, it would prove that the protoplasm of a multi- cellular plant behaves just like the naked, creeping plas- modium, which is also heliotropically irritable. III]. SUMMARY OF THE MECHANICAL EFFECTS OF LIGHT IN THE ANIMAL KINGDOM WHICH ARE THUS FAR KNOWN I shall in this chapter summarize briefly the facts and views in regard to the movements of animals under the in- fluence of light, so far as they are known up to the present time. These may be divided into three groups: 1. Casual observations of the older authors (Réaumur, Trembley). These are unprejudiced records of simple obser- vations. 2. Modern investigations on the effects of light from an anthropomorphic standpoint. The movements of animals are not attributed to mechanical causes, but to supposed human sensations of the animals. 3. Investigations according to the method of Sachs, which, however, have been applied only to Protozoa. The last- named observations are the most important in these three groups. The earliest account of the effects of light on animals which I have found in the literature is by Réaumur.’ He found that moths which are attracted by the candle flame “‘do not fly from flower to flower during the day.” Since he saw chiefly the males fly into the flame, he raised the question as to whether or not the female moths emit light like glow- worms. ‘Do not the females of the nocturnal Lepidoptera emit a light too feeble to make an impression on our eyes, but sufficiently strong to act on those of their males?” He had observed, evidently, that the males of the glow-worm which are attracted by the light to the aboral end of the females likewise fly into the light. Réaumur was, moreover, 1RBauMuR, Mémoires pour servir & Uhistoire des insectes, Vol. I, 1, p.330 (Amster- dam, 1748). Digitized by Microsoft® 8 STUDIES IN GENERAL PHYSIOLOGY convinced that the glow-worm living in the woods could see. He made a glass window in a tree in which such worms lived and noticed that the animals gave a start upon the approach of a burning candle. Trembley made far better experiments.’ He found that “water fleas”.can be driven around in a circle by a moving candle: By the light of a wax taper I observed polyps to which during the day I had given many water fleas; in the evening there were left in the glass some which the polyps had not consumed. I noticed that most of them had collected on the side toward the candle. I changed the position of the taper, and they followed it. As I had moved its position repeatedly, and each time had seen that the water fleas followed it, I moved the taper slowly around the glass without stopping. They followed, and thus made several trips around it. I have had the opportunity of repeating this ex- periment several times. Trembley’s observations on the effect of light on Hydra were made with great care. After he had repeatedly observed that the polyps moved to the “brightest” side of the glass, he placed ‘‘a glass containing many green polyps in a case which had an opening on one side about opposite the middle of the glass.” He reports as follows concerning their behavior: When I placed the glass so that the opening in the case was turned to the light, the polyps always migrated toward that side of the glass which was opposite this opening, in such a way that together they made the figure of a gable. I often turned the glass around, and after several days I observed the polyps again at the opening arranged as before (in the form of a gable). To vary the experiment still further, I fixed the dark case so that the open- ing was at times straight, at other times inverted, and again the polyps arranged themselves together. After he had discovered that polyps which had been cut in two could “move, eat, and multiply,” he tried to see “whether 1TREMBLEY, Abhandlungen zur Geschichte einer Polypenart, transl. by GOTzZE (Quedlinburg, 1791). Digitized by Microsoft® HELIOTROPISM OF ANIMALS 9 these pieces would turn toward the light in the same way as the undivided polyps.’ He cut a number of polyps in two: the anterior halves he placed in one glass, the posterior halves in another. He found “in oft-repeated experiments that the animals in both glasses collected in the brightest regions in the glass.” These are, as far as I know, the only extended observa- tions to be found in the old physiological literature of the effects of light upon animals. For a long time no further study of the effects of light upon animals was made. Johannes Miller mentions, in the preface to his Physiologie des Gesichtssinnes, that he made “investigations on the in- fluence of colored light on the vital phenomena of plants and animals,” but, as far as I know, the results of his investiga- tions were never published. The modern anthropomorphic observations were intro- duced by Paul Bert. Bert raised the question: Do all animals see the same rays that we see?’ He meant to ask whether all rays of the visible sun spectrum are able to bring about animal movements. An experiment with Daph- nia pulex was sufficient for Bert to settle this question. He projected a spectrum and found that the animals became restless in all positions of the visible spectrum: Mes daphnies erraient dispersées d’une maniére & peu prés égale dans toute |’étendue du vase obscur, lorsque soudain je fis tomber sur la fente un rayon coloré, un rayon vert. Aussitét elles s’agitérent, se groupérent toutes dans la direction de la trainée lumineuse et un trés-grand nombre s’en vint se heurter, montant et descendant sans relache contre la paroi qui recevait la lumiére. Or, un semblable résultat fut obtenu pour toutes les régions du spectre visible. Le rouge, le jaune, le bleu, le violet méme atti- raient les daphnies. Seulement il fut facile de remarquer, qu’elles accouraient beaucoup plus rapidement au jaune ou au vert qu’a toute autre couleur. 1BERT, Archives de physiologie, 1869. Digitized by Microsoft® 10 STUDIES IN GENERAL PHYSIOLOGY On either side of the spectrum the animals remained at rest. In addition to this, Bert made another experiment. He had a spectrum projected on a trough, and observed how the animals distributed themselves over the different parts of the spectrum. L’immense majorité se placa dans le jaune, le vert, l’orange; une assez grande quantité se voyaient encore dans le rouge, un certain nombre dans le bleu, quelques-unes de plus en plus rares & mesure qu’on s’éloignait dans les régions plus réfrangibles du violet, au dela du rouge, au dela de l’ultra-violet; dans les régions invisibles, en un mot, on n’en trouvait que d’isolées en promenade accidentelle. From these facts Bert concluded that Daphnia behaves in the spectrum much as a man would, who, when reading a book, would move into the brightest part of the spectrum, into the yellow light. Lubbock repeated Bert’s experiment on Daphnia.’ One- half of a dish was covered by a yellow screen; the other half was left uncovered. In the uncovered half 1,904 animals collected, while 3,096 gathered under the yellow screen. From this Lubbock concludes that Daphnia has a “preference”’ for ‘‘yellow.”” But one would suppose that in the uncovered part of the dish there was at least as much yellow light as under the yellow screen; or did the majority “hate” the blue light? When Lubbock covered one-half of the trough with blue glass and left the other uncovered, he found 2,046 animals under the blue glass, and 2,954 in the uncovered part of the trough. Whether one is to conclude from this that blue light is in the sense of Lubbock “disagreeable” to Daphnia is not stated. When half of the trough was covered with red glass, there collected 1,928 animals under the red glass, while 3,072 collected in the uncovered por- 1 Lussocs, “Die Sinne und das geistige Leben der Thiere,” Internationale wissenschaftliche Bibliothek, Vol. LX VII (1889). Digitized by Microsoft® HELIOTROPISM OF ANIMALS 11 tions of the dish. When half of the vessel was covered with an opaque porcelain screen, Lubbock found 2,048 animals collected under it, and 2,932 animals in the un- covered half. From these and similar experiments Lubbock concludes that the animals have a decided preference for yellow light. I also have made some experiments on the effects of rays of different refrangibility on Daphnia, and found that when the more refrangible rays (blue and violet) fell upon the animals they hastened to the source of light and moved up and down on the light side of the vessel. When I made the same experiment with the less refrangible rays, the effect was weak or did not take place at all. The result conforms with other facts which are to be described later. I shall, therefore, not revert to the Daphnia and their alleged ‘‘preference for yellow.” Lubbock has employed a similar method in his experi- ments on wingless ants;' these, however, led to much more fruitful results than his experiments on Daphnia. In an experiment in which a vessel was covered with strips of red, green, yellow, and violet glass he found that 890 animals collected under the red glass, 544 under the green, 495 under the yellow, and only 5 under the violet. There is no doubt in this case that the animals collected under those glasses where they were struck by the less refrangible rays. Other experiments showed that red glass acts like an opaque body. The observation of Lubbock that ants avoid the ultra- violet part of the spectrum is also worthy of note. For the sake of completeness the experiments of Lubbock on bees and wasps must be mentioned, in which it was found that under otherwise similar conditions blue objects smeared with honey were preferred to those of another color. 1 Luspock, “ Ameisen, Bienen und Wespen,” ibid., 1883. Digitized by Microsoft® 12 STUDIES IN GENERAL PHYSIOLOGY The most extended experiments on the influence of light on the orientation of animals were made by Graber." His “comparative studies on light-sensations” (Vergleichende Licht-Gefihl-Studien), as he called his investigations, cover about fifty species. His method is that followed by Lub- bock. The faultiness of this method and the errors of interpre- tation of the results obtained stand out more clearly in Graber’s writings than in Lubbock’s. Graber covers one- half of a vessel with a partially or completely opaque screen, and after a time notes how the animals are dis- tributed in the vessel. If most of the animals are under the opaque screen, Graber says that they are ‘“‘fond of the dark” and “hate the light;” or in the reverse case, that they are ‘‘fond of the light” or of “the white” and “hate the dark.” He therefore uses the conceptions of “white” or “bright” and “dark,” which designate certain effects of light upon a human being for the conceptions of great or small intensity of the light; and in saying that animals which “prefer the light’’ also “hate the darkness” he makes a second mistake in that he maintains that strong and weak light have opposite effects. We shall see, however, that these effects are similar and differ only in degree. He makes the same mistake in experimenting on rays of different refrangibility. The most important among the facts ob- served by him in this connection is this, that animals which “prefer the light” with a few exceptions also “prefer” blue, while those which “hate the light” “prefer” red. His ideas are expressed in the following remarks, which, however, I do not fully understand: The question arises as to the cause of this truly striking rela- tion between the love for white light and for blue light, on the one hand, and between the dislike for white light and for blue light, 1Grundlinien zur Erforschung des Helligkeits- und Farbensinnes der Thiere (Prag, 1884). Digitized by Microsoft® HELIOTROPISM OF ANIMALS 18 on the other hand. If the law had reference only to white light, and not also to colored light—red, blue, etc.— which is, however, by no means always the case, one might at first be inclined to be- lieve that the animals which prefer red avoid mixed light because it contains many of the hated short waves of the blue and violet light; for this very reason it would be more agreeable than dim light to the animals which prefer blue, for dim mixed light is poor in all rays, and therefore also in blue. Yet the objection might be raised against this explanation that mixed light contains as much red for those animals which prefer red as it contains blue for those animals which prefer blue. Yet this objection could again be weakened by the assumption that, since the animals which prefer red also prefer darkness, they prefer a minus of their chosen color to a plus of the color they dislike. Graber finally considers it best “‘to await further investi- gations in a field where great darkness still prevails.” We see that Graber in regard to the effects of monochromatic light again establishes a contrast in effects where, as we shall see, a similarity exists. Graber was prevented from cor- rectly interpreting his results by attributing the movements of animals to sensations instead of to physical causes. If he had given up the anthropomorphic standpoint, he would soon have discovered that his experiments show that the more refrangible rays are more effective in causing the orientation of an animal than the less refrangible ones. In none of the investigations of Bert, Lubbock, or Graber has the influence of the direction of the rays on the orienta- tion been studied. Graber, for example, took it for granted that an animal moves to the light because, as he expressed it, “it is fond of the light” or “the white.” If it moves in the opposite direction, it “is fond of the dark.” Lubbock remarks incidentally that “ants do not like light in their nests, probably because they do not deem it safe.” This sums up the opinions and results of the authors who sought to explain anthropomorphically the phenomena which interest us here. Digitized by Microsoft® 14 STUDIES IN GENERAL PHYSIOLOGY Finally, I have to mention the heliotropic investigations on Infusoria which were made along the lines mapped out by Sachs. To bring these investigations before the reader I shall describe the more important observations which have been made on Euglena. The influence of the direction of the rays of light on these Infusoria was first demonstrated by Stahl:? Those individuals which did not swim about freely remained with their pointed posterior ends attached to the cover-glass or to other objects, while their free anterior ends were, according to con- ditions, either turned toward or away from the source of light. The longitudinal axes of both the motile and sessile Euglenz coincided as nearly as possible with the direction of the rays of light. The motionless ones behaved like the free-swimming ones whenever the direction or intensity of the light was suddenly changed, except that they reacted more slowly. If, for example, the glass slip was suddenly rotated through an angle of 180°, the position which the animals occupied originally with reference to the source of light was slowly reassumed, while the swimming individuals left their former path and moved in the original direction toward the light immediately after a change in its direction. Engelmann studied in Euglena the relation between the effect of the rays of light and their refrangibility. After he had established the fact that when a drop of Euglenz is only partially illuminated the animals gradually accumulate in the lighted area, he brought the animals into a micro- spectrum. Here they collected on the more refrangible side of the spectrum. The orientation of Euglena therefore depends on the direction of the rays, and especially on that of the more refrangible ones. It must finally be men- tioned that the anterior ends of the Infusorie are most sen- sitive to light; yet the pigment spot is not, as might be supposed, the most sensitive, but the colorless protoplasm in front of this. Besides these direct effects of light in phenomena of 1 Botanische Zeitung, 1880. 2 Pfliigers Archiv, Vol. XXIX (1882), Digitized by Microsoft® HELIOTROPISM OF ANIMALS 15 orientation, which alone interest us here, there are also cer- tain indirect effects on the orientation of low forms of life. These were also first observed by Engelmann. When the supply of oxygen is cut off from certain chlorophyll-bearing organisms, they remain in that part of the spectrum in which assimilation takes place. In water with its normal amount of oxygen, as Engelmann found, Stentor viridis, Bursaria, and the green slipper animalculz do not react to light.’ If, however, the supply of oxygen from without is interfered with, “the insufficient supply can be compensated for by a production of oxygen by the chlorophyll granules within the mesoplasm.” Under these conditions the animals return to the light side of the drop when they accidentally get into the shady part. When the animals are brought into a micro- spectrum, they collect in those regions which promote assimi- lation. The opposite effect takes place, however, when the supply of oxygen from without exceeds the normal. When Engelmann passed a stream of pure oxygen through the water, the animals moved from the lighted into the shaded part of the drop. Such an indirect orientation toward light as is determined by assimilation is shown also in the behavior of the purple bacteria.” These, as Engelmann found, collect in those regions of the spectrum which are. most absorbed by the coloring matter of the bacteria. These are the most important facts which up to this time are known concerning the influence of light on the orienta- tion of animals. Thus far only the observations made on Infusoria are sufficient to warrant the conclusion that ani- mal movements depend on light in the same way as_ the movements of plants. In the rest of the animal kingdom either the facts necessary for this conclusion are lacking, or false statements and conceptions are prevalent. So far as 1 Tbid., p. 387. 2 ENGELMANN, Botanische Zeitung, 1888. Digitized by Microsoft® 16 STUDIES IN GENERAL PHYSIOLOGY the latter are concerned, it is wrong, as we shall see, to say that certain animals ‘‘are fond of the light” and seek those regions in space where light is most intense, while others “are fond of the dark” and betake themselves to those regions which are darkest. In contradiction of this idea I shall prove that the direction of the progressive heliotropic move- ments of animals is determined solely by the direction of the rays, no matter whether the animals move from regions in which light is less intense to those in which it is more intense, or vice versa. Further than this, it is fundamentally wrong to say that an assumed ‘‘preference for color” determines the orientation of animals toward rays of different refrangibilities; that, as Graber says, the animals which ‘are fond of blue” “hate red,” and that those which “are fond of red” ‘‘hate blue.” In contradiction of this idea I shall prove that there are no animals which “are fond of” red or “hate” blue, but only such as move toward a source of light or away from it; and that these movements occur in the same way under the influence of the more refrangible rays as under that of the less refrangible rays, only with this purely quantitative difference, that the more refrangible rays, as in plants, are much more effective than the less refrangible ones, which usually have no effect. I consider it inadvisable to represent the movements ob- served in animals as the expression of a “color preference,” or a “color sensation,” of a “pleasurable” or ‘“unpleasur- able sensation,” as do most animal physiologists and zoélo- gists who have studied the effects of light in the animal kingdom. I do not propose to base an analysis of the movements of animals on such hypothetical, anthropomorphic sensations and feelings, but on such conditions as determine the course of phenomena in inanimate nature as well. Real natural science began when, instead of fabulizing over the Digitized by Microsoft® HELIOTROPISM OF ANIMALS 17 nature of gravitation, men determined accurately the details of the movement of falling stones, of pendulums, etc., and described them in the most simple ‘and definite terms. In biology, especially in regard to the mechanical effects of light which concern us here, the task of the investigator can only be to determine and describe the circumstances upon which depend the movements of animals under the influ- ence of light. Iv. REMARKS ON THE METHOD OF EXPERIMENTATION.— THE HELIOTROPISM OF AN ANIMAL USUALLY BECOMES EVIDENT ONLY AT A DEFINITE EPOCH IN ITS EXISTENCE.—THE HELIOTROPISM OF AN ANIMAL CAN EASILY BE OBSCURED BY A SPECIAL FORM OF CONTACT-IRRITABILITY The facts which‘I have to prove are so simple that almost all technical apparatus can be dispensed with. If one attempts to demonstrate that the orientation of the animals is controlled by the direction of the rays of light, care must be taken that light falls upon the animals from only one side. To accomplish this it is sufficient to carry on the experiments in a room which is lighted from one side only. Since the animals with which we are dealing in this discussion are dorsiventral and place their median planes in the direction of the rays of light, progressive movements are possible in only two directions—either toward the source of light (when they will be called positively heliotropic), or away from the source of light (in which case they will be called negatively heliotropic).’ Diffuse daylight was used as the source of light, and only where specially mentioned was sunlight employed. 1 Some botanists designate the movements of motile plant organisms toward a source of light as ‘ phototactic,” in contrast to the ‘‘heliotropic” movements of sessile plants. Since the observations of Sachs, Stahl, and Wortmann, however, leave no room for doubt that the processes are identical in both cases, it seems tome that this separation is not justified. Otherwise a “ phototactic” animal ought to become “heliotropic’? when its progressive movements are prevented. For this reason I use the same term for similar processes, (See WORTMANN, Botanische Zei- tung, 1887.) Digitized by Microsoft® 18 STUDIES IN GENERAL PHYSIOLOGY There are two methods by which the second fact, that only the more refrangible rays bring about orientation, can be proved, namely, by experimenting with prismatic spectra or with colored screens. All authors who have studied the behavior of plants behind colored screens have obtained the same result—that it is only, or more especially, the more refrangible rays which are heliotropically active. Studies on the behavior of plants in prismatic spectra have led to harmonious results, in so far as they confirm the gross results obtained by using colored screens; yet opinions differ as to the efficacy of the more limited portions of the spectrum. Since for the present I wish to show only that the laws governing the orientation of an animal toward light correspond to the laws governing the orientation of plants toward the same stimulus, it was necessary to use as a basis the really established data of plant physiology, and I therefore shall confine myself to the proof of the fact that the more refrangible rays of the spectrum are exclusively, or almost exclusively, effective. To do this I proceeded as is usual in plant physiology. In order to have only the less refrangible rays act on the animals, I passed the diffuse daylight through a solution of potassium bichromate or ruby glass; to study the influence of the more refrangible rays, I chose cobalt glass or an ammoniacal solu- tion of copper. The screens were examined spectroscopically. The dark-red glass which I used completely absorbed the more refrangible rays, and let through only the red, yellow, and a part of the green rays. The dark-blue glass absorbed the less refrangible red and yellow and a part of the green rays, with the exception of a small region in the outer red. Since, however, the heliotropic phenomena appear only weakly or not at all behind dark-red glass, while they occur just as in diffuse daylight behind dark-blue glass, the few red rays which penetrate the dark-blue glass cannot be Digitized by Microsoft® HELIOTROPISM OF ANIMALS 19 responsible for the heliotropic phenomena which take place so energetically behind this screen, but can be due only to the activity of the more refrangible rays. The other external conditions which must be considered in heliotropic investigations are so simple that they do not call for any special explanations. Where they are of impor- tance they will be self-evident. It is very essential, however, to realize that the helio- tropism of an animal often manifests itself clearly only dur- ing a definite, often decisive, period of its existence, only to diminish again or to disappear entirely later. 1t was only by observing for weeks and months the animals described in * this treatise, which for the most part I raised myself, that I have been able to establish this fact. The caterpillars of Porthesia chrysorrheea, for example, are energetically positively heliotropic only during a certain period of their existence, when they have just left the coc- coon in which they have wintered, and have not yet taken food. At this time the entire existence of these animals is a function of the light. Under natural conditions they hatch out on a warm spring day. The light compels them to creep to the tips of the branches, where they find their first nourishment in the young buds. When fed they are still positively heliotropic, but very much less so than before. If anyone should examine them in this condition, he would scarcely pronounce them heliotropic. It is not, however, a certain date of the year which gov- erns this heliotropism; for whenever I forced the animals to leave their nest (by raising the temperature), whether at the beginning of summer or of winter, they were indefatigable in their attempts at creeping toward the source of light. Winged ants are pronouncedly dependent on light only at a definite period of their existence—at the time of their nuptial flight. The same animals which were actively helio- Digitized by Microsoft® 20 STUDIES IN GENERAL PHYSIOLOGY tropic at the time of the nuptial flight were practically indifferent toward the light a few days previously. In the same way, later on their heliotropism was entirely pushed aside again by another form of irritability, frequently encountered in the animal kingdom, and to which I shall soon return. Fly larvee also possess very different forms of heliotropic irritability at different epochs in their existence. Negative heliotropism is not very distinct in the newly hatched larve; but the animals turn their ventral surfaces toward a suffi- ciently intensive source of light without otherwise being influenced by the direction of the rays of light. Full-grown larve, however, place their median planes very sharply in the direction of the rays of light, provided the light is suffi- ciently intense. I believe that this periodic appearance of heliotropic irritability plays a great réle in the ecology of animals. The periodic migrations of many animals, such as birds of passage, might be explained in this way. It is a well-known fact that the irritability of an animal in the larval stage may be entirely opposite in kind to that of the adult stage. This phenomenon is very common. The larva of the fly is negatively heliotropic, while the imago is positively heliotropic; this is also the case with June-bugs and many other animals. I encountered this inversion of the sense of heliotropism when the animal changed from the larval stage to the mature state so frequently that for a time I thought it a universal rule. Such, however, is not the case. Caterpillars, for example, behave toward light as does the imago, as I know from my own experience and from what I can find on the subject in the literature. The behavior of an animal is determined by the sum of all the forms of its irritability. The heliotropic irrita- bility, therefore, may be obscured by a more powerful irrita- bility of another sort. This is often due to a special kind Digitized by Microsoft® HELIOTROPISM OF ANIMALS 21 of contact-irritability, which, so far as I know, has not yet been recognized. Many insects are compelled to bring their bodies in contact with the surfaces of solid bodies in a very definite way. My attention was called to this phenomenon in my experiments on animal geotropism, in which I allowed the animals to move about on geometrically simple bodies bounded by plane surfaces. I noticed that the animals rarely remained on the plane surfaces, but collected about the edges, particularly the vertical ones. It is worthy of note that certain animals always seek the concavity of the angle between the sides of hollow cubes, while others just as con- stantly move on the convex side. The caterpillar of Por- thesia chrysorrhcea is an example of the latter type. The other form of this contact-irritability, which leads the ani- mals to the concavity of the angles, is very common. The following observations show how this form of irritability might easily be confused with the irritability toward light, and so lead to a misconception of the behavior of the animal toward light. I studied for several weeks a large number of moths of the species Amphipyra. The animals are remarkable in that they are more given to running than to flying. The rapidity of their running movements calls to mind the lively movements of cockroaches and ants. While formerly I had found that all butterflies are positively heliotropic, I observed that Amphipyre when let loose, did not fly to the window, but to the nearest wall or to the floor, where they ran about nimbly and crept under the first suitable object, like cock- roaches. This looked as though the animals fled from the source of light. Yet it could be shown that the animals move toward a source of light, and that the inclination to creep intocrevices depends upon the contact-irritability, which was mentioned before. The following experiments always succeeded: In the evening, when a lamp was brought into Digitized by Microsoft® 22, STUDIES IN GENERAL PHYSIOLOGY the neighborhood of a box containing the animals, those which reacted at all always flew with great violence to the side of the vessel which was turned toward the light. In no case did they fly in the opposite direction. The experiment was unequivocal and could be interpreted in but one way. So far as the contact-irritability is concerned, the animals collected in the four concave vertical edges when kept in a cubical wooden box, which was covered on top with window glass. In this position they assumed an indifferent orienta- tion toward the source of light. To make perfectly sure of this fact, I employed the following method: I placed a plate of window glass so close to and parallel with the plane of the floor of the vessel containing the animals that they could just wedge themselves in between the floor and the window glass. The glass plate was entirely exposed to the light. Those animals which by chance came to the edge of the glass plate crept under it, and remained in this position exposed to the light, in contact, however, both above and below with solid bodies. On the next day all the animals were under the glass plate. The animals are therefore forced to bring their bodies in contact with other solid bodies, and it is this (and not the light) which causes them to creep under solid bodies. I placed a ball of paper in the vessel containing the animals; a part of them crept under the paper and a part into its folds. In nature these butterflies remain in the clefts on the bark of trees or on the ground in mead- ows. Forficula auricularia are found in great numbers in verti- cal crevices (such, e. g., as the spaces between gate and gate- post, in the entrance to gardens). I obtained the animals for my experiments by hanging a cloth of cotton on the top of a small grape vine. The animals collected in the folds of the cloth. These animals in reality move away from the light ; that is to say, they are negatively heliotropic ; but it Digitized by Microsoft® HELIOTROPISM OF ANIMALS 23 would be wrong to attribute their tendency to creep into the folds of the cloth to their negative heliotropism. When I experimented on these animals with the glass plate, I found that they wedged themselves under it, and remained there exposed to broad daylight, rather than creep away from it. Inside of a box the animals collected in the concave edges; and it was very noticeable that the animals rarely ran over the free surfaces, but nearly always along the edges, as if it were ever necessary for them to have their sides in contact with solid bodies. I believe that this form of contact-irritability is identical with the important phenomenon, observed by J. Dewitz," that spermatozoa are compelled to turn a certain side of their bodies toward solid bodies. Because of this contact-irrita- bility a spermatozoon is never able to leave a cover-glass or a glass slide when once it comes in contact with it. I have observed the same phenomena in hypotrichal Infusoria. These always turn one side of their bodies, the ventral, toward solid bodies. They further resemble the spermatozoa observed by Dewitz in that they alter the direction of their movement always in the same sense, so that on the cover- glass of a microscopical preparation are found only Infu- soria which move in one direction, while on the glass slide they seem to move in the opposite direction. In order to distinguish this form of contact-irritability f-om other forms of contact-irritability (such as the rolling up or progressive or retrogressive movements when touched), I shall call the peculiarity, possessed by some animals, of orienting their bodies in a definite way toward the surface of other solid bodies, stereotropism. The co-operation of other forms of animal irritability with heliotropism is so simple as to be self-explanatory wherever we may encounter it in our experiments. 1J,Drwitz, Pfliigers Archiv, Vol. XXXVIII (1886). Digitized by Microsoft® 24 STUDIES IN GENERAL PHYSIOLOGY Vv. THE POSITIVE HELIOTROPISM OF THE CATERPILLARS OF PORTHESIA CHRYSORRH@A I will enumerate the observations which show the identity of animal and plant heliotropism in the caterpillars of Por- thesia chrysorrheea. I shall mention only such experiments as in my experience were always successful under the given conditions, and which may be taken as the prototype of the experiments made upon all the animals treated of in this discussion. 1. The direction of the progressive movement in animals is determined by the direction of the rays of light.—I placed a large number—about a hundred specimens of the small gregarious caterpillars of Porthesia chrysorrhcea which had just crept out of the web in which they had passed the win- ter—into a test-tube. They had not fed as yet, and in this hungry condition they were exposed to the light. The tem- perature of the room was necessarily more than 12°-15° C., as otherwise they would have crowded together and fallen asleep again—a state in which they react neither to light nor to gravity. Experiment 1.—If the test-tube is laid on a dark table, so that the longitudinal axis of the tube is perpendicular to the plane of the window, the animals, which are at first scat- tered about irregularly, all assume the same orientation. They creep to the upper portion of the test-tube, turn their heads toward the window, and with their ventral surfaces and their heads turned toward the light creep in a straight line toward the window side of the test-tube. The process requires from one to five minutes, according to the tempera- ture and the condition of the hibernated animals. AJl with- out exception, provided they are not sickly, move in the direction of the rays of light to the window side of the test- tube. If the tube is turned about an angle of 180°, the Digitized by Microsoft® HELIOTROPISM OF ANIMALS 25 process is repeated, the animals creeping to the window side of the glass just as before. If, however, the position of the glass remains unchanged, the animals remain permanently crowded together on the window side of the test-tube. Experiment 2.—If the test-tube is laid on the table with the longitudinal axis parallel to the plane of the window, the ang gradually scatter uniformly over the whole of the upper part of the tube. The lower portion of the vessel is in x consequence again free from animals. If the longitudinal axis of the test- tube lies at even a slight angle with the plane of the window, the animals move to the end of the tube nearest the window, and remain there in their FIG. 1 customary position. Experiment 3.—The test-tube is placed perpendicular to the plane #' of the window, and at the beginning of the experiment the animals are collected at the window side B of the test-tube (Fig. 1). That half of the vessel which lies nearest the window is now covered with an opaque paste- board box, K. The following then occurs: The animals soon appear at A on the room side of the pasteboard box ; as soon, however, as they emerge from the box K into A, they turn about, direct their heads toward the window, move to the edge of the pasteboard, and remain at the boundary between the covered and the uncovered portions of the tube, at A and especially at the top of the test-tube. The remark- able thing is that they are not distributed evenly over the whole brightly illuminated part of the test-tube. The explanation is as follows: As soon as the animals near the window at B are covered by the pasteboard, the weak rays of light reflected from the walls of the room fall upon them. Digitized by Microsoft® 26 STUDIES IN GENERAL PHYSIOLOGY The animals follow the path of these rays and arrive at the uncovered portion of the tube. As soon, however, as the strong rays of diffuse light fall upon them at A, they turn about and direct their heads toward the window, until they come again under the pasteboard which shuts out the diffuse light. They are then again attracted by the light of the room, and so on, until they come to rest at the boundary between the two regions at A. At the beginning of the experiment, before the animals stop moving it can really be seen that they are driven around in a narrow circle. If at the beginning of the experiment the animals are collected, not on the window side, but on the room side of the test-tube at C, they move toward the window until they reach the pasteboard at A. If the tube is pulled away from the window for some distance, while the pasteboard remains stationary, the animals begin to move, until they reach the edge of the pasteboard. If the tube is placed horizontally with the longitudinal axis parallel to the window, the animals distribute themselves over the whole length of that portion of the tube which is not covered by the pasteboard, collecting, however, always on the window side of the tube. According to the prevailing views of zodlogists and ani- mal physiologists, the movement of caterpillars toward the light is determined by the animals’ “fondness for light.” They, therefore, move from a region of less intense light to one of greater intensity. That the essential feature, how- ever, is the direction of the rays, and not a difference in their intensity,’ is evident from the following experiments. Experiment 4.— The animals are in a glass cylinder a, some 38cm. in diameter. Light can enter it from all sides (Fig. 2). The inside of a second test-tube b, which has the 1Jn different parts of the tube. [1903] Digitized by Microsoft® HELIOTROPISM OF ANIMALS 27 same diameter, is covered with dull black paper, except for a strip about 2mm. wide. The two test-tubes are placed together on a table so that their longitudinal axes lie per- pendicular to the plane / of the window, and the transparent side cd of the glass b is turned up; the animals move along the illuminated side cd from a to b, with- out stopping at the boundary between them, until they reach the window side ¢ of the cylinder. The total amount of light which strikes a caterpillar in the glass b, however, is less than in the glass a, since all lateral rays are cut off in the former and the animal is struck by rays a of light only on its ventral side; in test- tube a light falls upon the animals from all sides, though the rays from above and in front are of course the most intense. The animals there- fore move toward the source of light in the direction of the rays of light, even if by so doing—to judge from human sensations—they are led from a “bright” to a “dark” place. In such an experiment no animals are found, as a rule, scattered over the rest of the surface of the glass b. If both glasses are turned around so that a is nearest the window side, the animals of course again move from b to «. The experiments described here were carried on in diffuse ° daylight. In sunlight, however, the results are the same as in diffuse daylight. When the glass is placed with the longitudinal axis in the direction of the rays, the animals move in the direction of the rays toward the sun and collect at the end of the glass which is turned toward the sun, even though in their hungry state they cannot bear the high tem- perature. When the test-tube is placed with the longitudinal axis perpendicular to the rays, the animals scatter over the FIG. 2 Digitized by Microsoft® 28 STUDIES IN GENERAL PHYSIOLOGY whole length of the tube, remaining, however, upon its sunny side. Orientation takes place more quickly in direct sun- light than in diffuse daylight. Experiment 5,—A small pencil SS of direct sunlight is allowed to fall on a table obliquely to the plane of the win- dow through the window F (Fig. 3). Rays of diffuse daylight fall upon the remaining portions of the table. If at the beginning of this experiment all the animals are at the end a of the test-tube —which is so placed on the table that a is in direct sunlight, while the other half 6 is in diffuse day- light, and is nearer to the plane a of the window than a—the fol- lowing occurs: The animals move from a through the pencil of direct sunlight into b, which lies in the diffuse daylight, where they remain at the cup of the test-tube. They pass from the direct sunlight into dif- fuse daylight without even attempting to return into the sunlight. This experiment can be explained only by the assumption that the orientation of the animals is determined by the direction of the rays. The animal can and must follow the rays of diffuse light which have the direction b—a. If, as is customary with zodlogists, we believed that these animals love the light—or, more correctly, that they prefer the more intense light—it would be impossible to see why they do not remain in the direct sunlight, or at least why they do not hesitate to go into the diffuse light. From what has been said, no one, I believe, will doubt that the direction of the progressive movements of the cater- Wy FIG. 3 Digitized by Microsoft® HELIOTROPISM OF ANIMALS 29 pillars of Porthesia chrysorrhea is determined by the direction of the rays of light, and not by differences in the intensity of the light in different parts of space. Positively heliotropic animals are compelled to turn their oral pole toward the source of light and to move in the direction of the rays toward this source. 2. The dependence of orientation on the refrangibility of the rays.—I shall now show that zt 7s the more refrangible rays of the visible spectrum which are chiefly concerned in bringing about the orientation of the caterpillars of Por- thesia chrysorrheea. Experiment 1.—If we place the test-tube on a table and cover it with a box of dark-blue glass, the animals behave as if the vessel were uncovered. Without exception, they move in a straight line to the window side of the vessel and remain there. If instead of blue glass we use red, which to our eyes seems much brighter than blue glass, no change occurs in the orientation of the animals at first; after a long time, however, the animals collect under the red glass on the win- dow side of the vessel. In direct sunlight, however, orienta- tion takes place more quickly. Exactly the same phenomena are observed if an ammoniacal solution of copper is sub- stituted for the blue glass, or a solution of potassium bichromate for the ruby glass. This is also true in the following experiments, where I may not always call special attention to it. This experiment shows (1) that the more refrangible rays have the same effect as mixed rays, and (2) that the less refrangible rays bring about movements in the same way as the more refrangible ones, only their effect is less intense. The experiment also proves that it is wrong to say, as do the anthropomorphists, that the animals ‘are fond of” blue and “hate” red; for, were this true, the animals should have been forced to move to the room side of the test-tube when under the red glass, yet they moved Digitized by Microsoft® 30 STUDIES IN GENERAL PHYSIOLOGY toward the window. The animals neither “are fond of” blue nor “hate” red, but they are like plants, simply positively heliotropic, and the blue rays are more effective heliotropically than the red. There is, as I shall state here once for all, no difference in direction between the movements called forth by blue light and red light; there is only a difference in the velocity and precision with which these heliotropic movements take place. Experiment 2.—The longitudinal axis of the test-tube is again perpendicular to the plane of the window. The small caterpillars are at the beginning of the experiment on the room side of the tube. The window half of the test-tube is covered with dark-blue glass. The experiment goes on as if the tube were uncovered; the animals move to the window side of the test-tube, where they remain under the blue cover. If the same experiment is repeated, only so that the blue cover is placed over the room side of the test-tube, the animals again move to the window, where they remain. The experiment proves that the more refrangible rays alone have the same effect as mixed light ; and the fact that the animals leave the uncovered portions of the test-tube to creep under the dark-blue cover corroborates what has already been said, that positively heliotropic animals move in the direction of the rays of light even when in so doing they pass from a place of greater intensity of light to one of less intensity. Experiment 3.—The test-tube again lies horizontally, with its longitudinal axis perpendicular to the window. At the beginning of the experiment the animals are on the window side of the test-tube. If the window half of the tube is covered with red glass (which may seem much brighter to us than the blue glass of the previous experi- ment), immediately after the red glass has been placed over the animals they appear on the room side of it, and collect at the boundary between the covered and uncovered parts of Digitized by Microsoft® HELIOTROPISM OF ANIMALS 81 the tube. If at the beginning of the experiment the animals were on the room side of the test-tube, they move until they reach this boundary. We therefore get the same results by using red glass that we got by using opaque pasteboard in a previous experiment. Taken together with the preceding ones, this experiment proves that pre-eminently the more refrangible rays of mixeddaylight are heliotropically effective. Although, as we have just seen, the rays passing through red glass or a red solution are not absolutely ineffective, yet the weak light which is reflected from the walls of the room, and which contains some blue rays, is more effective than the diffused light reflected from the sky after it is filtered through red glass. It is for this reason that the animals on the window side under the red cover migrate to the boundary of the red screen where they are held by the rays of diffuse daylight. Experiment 4.—If, as before, we place the test-tube with the longitudinal axis perpendicular to the window, and cover it with red glass on the window side and with blue glass on the room side, the animals collect under the blue glass at its boundary with the red glass. Experiment 5.—If we place the test-tube with its longi- tudinal axis parallel to the window, the animals scatter over the whole length of that part of the tube which is covered by blue glass. From all these experiments it follows that it is chiefly the more refrangible rays which determine the orientation of the caterpillars of Porthesia chrysorrhea toward light. The only difference between the heliotropism of these animals and the heliotropism of plants is this, that the less refrangible rays are not so completely imeffective in the case of the caterpillars of Porthesia chrysorrheea as they apparently are in many plants. This point must, however, be studied more accurately with the aid of a spectrum. Digitized by Microsoft® 82 STUDIES IN GENERAL PHYSIOLOGY 3. The dependence of the orientation on the intensity of the rays of light.—It is a peculiarity of all animal as well as plant structures that only external stimuli of a certain inten- sity can call forth reactions. It can easily be shown that at the approach of twilight there comes a time when the rays of diffuse daylight coming through a window no longer attract caterpillars of Porthesia chrysorrhcea. If the animals are between two sources of light of differ- ent intensities, that having the greater intensity is the more effective. This can easily be shown by bringing the animals into a room into which light enters from opposite directions. Other conditions being the same, the animals move to the window nearest them. A maximum limit for the intensity of the light cannot be established, as direct sunlight is in itself effective. Artificial sources of light above a certain intensity and containing the more refrangible rays affect the animals in the same manner as the natural sources of light. In a dark room caterpillars are attracted by a kerosene flame as markedly as moths; the caterpillars, however, are not burned, because they move so slowly that they have time to turn back before the zone of fatal temperature is reached. Such animals as are attracted by direct sunlight may also be attracted by the candle flame, exactly as is the case in posi- tively heliotropic plants. 4. At a constant intensity light acts as a continuous source of stimulation.— If the test-tube which is placed with its longitudinal axis perpendicular to the window is left undisturbed, the animals remain permanently on the side nearest the window. Under these conditions we can also safely open the room side of the vessel without a single animal changing its position or escaping from its cage. It is remarkable, however, that when the test-tube has been left undisturbed all day, the animals keep.their position during the night. In this way I have kept animals for several days Digitized by Microsoft® HELIOTROPISM OF ANIMALS 33 in a test-tube open on the room side; but when I turned the vessel through an angle of 180° in the daytime, hardly two minutes elapsed before all the animals had moved to the open end of the vessel which was now turned toward the window. Under these conditions they of course escaped from the test-tube. A position which the animals have assumed under the influence of light is usually not changed when the light is removed, unless some other stimulus comes into play. 5. On negative geotropism and contact-irritability in the caterpillars of Porthesia chrysorrhea.—The reader may perhaps have noticed that in all of these experiments on caterpillars the test-tubes were always placed with their longitudinal axes horizontal. This was due to the fact that the animals behave like plant structures, not only in regard to their heliotropic, but also in regard to their geotropic, irritability. Just as is frequently the case in positively heliotropic plants, we find that the caterpillars are also nega- tively geotropic; that is, they are compelled by gravity to creep vertically upward until they come to rest in the highest part of the test-tube. These experiments were made in a dark room, with the long axis of the test-tube in a verti- cal direction. If the test-tube is inverted, the animals again ereep to the top; if left undisturbed, the animals remain in the uppermost regions of the test-tube. It is necessary in these experiments, as in those on heliotropism, to have the temperature of the room at least 15°, preferably as high as 20-22°. It is simplest to put the test-tube in one’s pocket with its longitudinal axis vertical. In a few minutes the animals are found at the highest point in the tube. An increase in temperature increases the geotropic irritability of the animals. it must now seem questionable whether in our former discussion of the heliotropism of these animals we were Digitized by Microsoft® 34 STUDIES IN GENERAL PHYSIOLOGY justified in taking as the effect of light the movements of the animals to the top of the test-tube ; it might, indeed, be a geotropic phenomenon. To decide this the animals were placed in a test-tube which was lined with thick black paper except for a strip 2 mm. wide. The uncovered strip was turned downward, so that light could enter the vessel only from below. Diffuse daylight was reflected through the slit from below by means of a mirror. The animals collected in the lower, lighted portion of the glass vessel. Their helio- tropism is therefore more powerful than their geotropism, even when only weak diffuse daylight is used. The geotropic experiments succeed only when the animals have been in the light for some time and have not yet come to rest. When the animals are kept in the dark for a long time and the test-tube is not disturbed, they do not creep upward. The orienting effect of the light always exceeds that of gravity. The effects of gravity, like the effects of light, usually appear only during certain periods in the life of the animals; at any rate, they cannot always be demon- strated with certainty. The contact-irritability of the caterpillars of Porthesia chrysorrhcea shows itself by the way in which the animals remain in the corners and convex sides of solid bodies. I covered the boxes in which I cultivated my caterpillars with large, square glass plates. These did not close the box tightly, so that the animals could creep out and creep upon the glass plates. Only rarely, however, were they found on the free surface of the plates. The animals moved along the rough edges of the plate until they reached the window side of the dish. I confirmed this observation almost daily for months. When I placed the animals upon the outside of a cubical block, they collected by hundreds in one of the upper corners. Of course, only a few have room in the corner itself, but, as is generally the case with these ani- Digitized by Microsoft® HELIOTROPISM OF ANIMALS 35 mals, when a few have collected in a spot the others on arriving hold fast to the sides of those already there. An animal at:rest acts upon a creeping one as a convex edge. On the other hand, I have never observed that the animals within the cubical box collect on concave edges. From this it follows that the friction of gliding over the convex corners is the source of the stimulation which compels the animal to come to rest there; in moving over the concave corners this friction, of course, does not take place. These three forms of irritability control mainly the daily life of the animals. We find them in great numbers in fruit trees and bushes, where they pass the winter in their nests; as soon as the warm weather comes, they leave their nests. Positive heliotropism and negative geotropism com- pel them to creep upward to the tips of branches, and contact- irritability holds them fast on the small buds. We can easily show that neither smell nor a special mystical “instinct” leads the animals to the buds, as we are able to compel them by the aid of light to starve in close proximity to food. The animals move to the window side or to the top of a test-tube in which they are kept. If then a branch covered with buds is pushed into the test-tube on the room side, the animals nevertheless remain where light and gravi- tation have compelled them to go and are holding them. If, however, they once are on the buds, the latter act as a stimulus which may be even stronger than the light. It is in such a case impossible to draw the animals away from the food by means of light. All these forms of irritability can best be demonstrated on animals which have just left the nest in which they have spent the winter, and which have not yet eaten anything. As soon as they have eaten and are about to moult, their irritability decreases, and at the time of moulting it is almost impossible to show any effect of light or gravity upon them. Digitized by Microsoft® 36 STUDIES IN GENERAL PHYSIOLOGY 6. The effect of temperature on the caterpillars of Por- thesia chrysorrheea.—The caterpillars of Porthesia chry- sorrhoea behave toward a source of heat in a manner opposite to that in which they behave toward light; they move away from the source of heat. If the animals contained in an opaque vessel are brought in the neighborhood of a hot stove, they leave the side of the vessel which is nearest the stove. Yet the heat does not compel the animals to move in a straight line, as they do when struck by the more refrangible rays of light. This directing effect of the more refrangible rays of the visible spectrum is greater than that of the dark heat rays. In this way it is possible for the same animal which flees from the source of the-dark rays of heat nevertheless to move in the direction of the sun’s rays to the sunny side of a vessel. It is a well-known fact that irritability in a tissue is a function of the temperature. I have already mentioned that at a temperature of less than 13° C. the animals are no longer affected by light. It can be shown that heliotropic irrita- bility increases with an increase in temperature. If the animals are kept during the day in a room having a tem- perature of about 18°, it is found that they no longer respond to light when beyond a certain distance from the window. If, however, the temperature of the test-tube is increased a few degrees, the animals move the more quickly to the win- dow side of the tube the higher the temperature. It can easily be demonstrated that the orientation takes place more rapidly, and that the direction of the progressive movements coincides more nearly with the direction of the rays of light, whenever the temperature is raised. If, however, the tem- perature is increased to 30° or over, the animals become very restless; they raise the anterior ends of their bodies higher than is usual in their movement, and so decrease the velocity of their progressive movements. The most suitable tem- Digitized by Microsoft® HELIOTROPISM OF ANIMALS Byi perature for demonstrating their heliotropic activity lies between 20° and 30°. The experiments on the caterpillars of Porthesia chry- sorrheea are typical. I have repeated them on some hundred species of insects, but I have never found a positively heliotropic insect whose dependence upon light was of a different kind from that found in Chrysorrhca. This fact has given me the impression that all animal proto- plasm, as perhaps all plant protoplasm, is heliotropically irritable, and that where this is apparently not the case the heliotropic reaction is inhibited, either temporarily or permanently, by other causes. For this reason it would be useless to publish here every single experiment I have made. This would result in repeating each time the same phenomena, only under the name of a different insect. Since there are only negatively and positively heliotropic animals, it would be of secondary interest to know to which of the two classes the individual animals belong. But I believe it necessary to show by concrete examples what part heliotropism plays in the habits and ecology of animals. VI. THE POSITIVE HELIOTROPISM AND THE SLEEP OF BUTTERFLIES Our knowledge of the behavior of butterflies toward light has, on the whole, remained at that point which is marked by the statement of Réaumur that “‘it is a singular fact that those butterflies which shun the daylight are pre- cisely those which fly into lighted chambers.”’ The paradox has not yet been explained why those butterflies which are not to be:seen by day fly into the flame at night, while the day butterflies apparently do not possess the tragic “instinct” of the night Lepidoptera. There is no lack of conjecture on this point. Romanes believes that the lamp is a “strange object” to the moths, and that ‘the desire to examine this Digitized by Microsoft® 88 STUDIES IN GENERAL PHYSIOLOGY strange object’’ drives the moths into the flame. We find, however, that the caterpillars of Porthesia chrysorrheea creep as well toward the sun as toward a lamp. Yet, according to Romanes, the sun ought to be a familiar ob- ject to these animals. Such anthropomorphic opinions as those of Romanes are evidently as useless in the analysis of life-phenomena as the speculations of metaphysicians — e. g., Hegel’s—on physical phenomena. A scientific analysis of the behavior of moths toward light leads to a very simple explanation of the paradox. Experiment 1.—Specimens of Sphinx euphorbize, Bom- byx lanestris, and other moths are kept in a large glass box. The box is placed in a room into which only daylight and no artificial light enters. As soon as the animals begin to fly, at the approach of twilight or later, they collect at the window side of their boxes. Whenever the box is reversed the animals fly back to the window side. This experiment is rendered more complete by the following observations: I kept the pupe of moths in an open box. Most of the moths hatched at night. On the following morning I always found them collected at the closed window of the room. Here they remained all day exposed to the light. Finally, when I caused the moths to fly by day, I noticed that they flew to the window as do all other positively heliotropic insects. These experiments show that the animals are attracted, not only by a lamp, but also by diffuse daylight. They also show that Réaumur’s idea that moths shun daylight is wrong. The experiments indicate that the animals are positively heliotropic toward diffuse daylight, although, as we shall soon see, this positive helio- tropism may during the daytime be obscured by another form of irritability. Experiment 2.—I brought some specimens of Sphinx euphorbie into a room which had a window only on one Digitized by Microsoft® HELIOTROPISM OF ANIMALS 89 side. On the wall of the room opposite the window I placed a kerosene lamp. At the approach of twilight, when the animals began to fly about, I brought them into the middle of the room, so that they were equidistant from the lamp and the window, and left them alone. They flew to the window. Yet, when I brought them into the immediate neighborhood (within about a meter) of the lamp, they flew into the flame. I repeated this experiment and convinced myself that they always flew to one of the two sources of light, either the window or the lamp; to the latter, however, only when they were in its immediate neighborhood. This experiment shows that the animals do not even pre- fer artificial to the natural light, but that the artificial light attracts them only when its intensity is greater than that of the diffuse daylight, which is the case at night when the animals are within a certain distance of the lamp, varying with the intensity of the flame. The heliotropic sphere of attraction of an electric arc light is therefore larger than that of a candle flame, and the number of moths attracted by it correspondingly greater. Experiment 3.—It must yet be proved that it is chiefly only the more refrangible rays of light which determine the movements of the moths. I studied the behavior of Sphinx euphorbie, which began to fly at about 9 o’clock in the evening. The animals were contained in a large box, 40 cm. long, the upper wall of which was of glass. Whenever I turned the box the animals at once flew to the window side and crowded against the upper glass wall through which the light came. When I placed a red glass over the window side of the box, the animals at once flew to the room side. They collected at the edge of the red glass, but on the room side of it, where they were not covered by it. Here they attempted to fly upward. When I used blue glass instead of Digitized by Microsoft® 40 STUDIES IN GENERAL PHYSIOLOGY red, they flew under it to the window side of the box. At fifteen minutes past 9 o’clock they came to rest and no longer reacted to light. When exposed to daylight on the follow- ing day, they did not stir, and made no attempt to creep away from the light, although sufficient opportunity was offered. I repeatedly established the fact that the movements of night butterflies are determined by the more refrangible rays of the spectrum on other specimens of Sphinx euphorbie. It was therefore not to be expected that in lamplight any other than the more refrangible rays would bring about movements. JI have convinced myself that the moths of Geometra piniaria are readily attracted by the light of a lamp when behind blue glass, but not when behind red glass. The night butterflies, therefore, shun neither diffuse nor intense light, nor do they prefer artificial light to diffuse daylight; the correct expression of the facts is rather this, that most species react to light only at night, when they are positively heliotropic like the day Lepidoptera. We find in butterflies periodic variation in irritability (as in many plants), and these variations correspond to the changes of day and night. As certain flowers open their calices only by night, while others open theirs by day, so certain butterflies fly only by day, while others fly only by night. Both classes of butterflies, however, are positively heliotropic; and it seems as if the irritability of the night butterflies toward light is not less, but even greater, than that of the day but- terflies ; for the intensity of the light which causes heliotropic phenomena in moths is apparently much less than the mini- mal intensity which stimulates day butterflies to heliotropic movements. The phenomena of sleep in butterflies are perhaps more complex than the corresponding phenomena in plants. One thing is, however, certain—that the periodicity of the noc- turnal movements of butterflies does not change during the Digitized by Microsoft® HELIOTROPISM OF ANIMALS 41 first two or three days if the animals are kept in the dark. Under these circumstances the moths become restless at the usual time. Réaumur showed that moths begin to fly in the evening when kept in a box. I must, leave it undecided for the present whether this periodicity finally disappears if the animals are kept still longer in the dark. I have tried repeatedly to cause Sphinx euphorbie to fly in the daytime by a sudden diminution in the intensity of the light. When I protected the animals from all jarring J never succeeded between 6 and 12 o'clock in the morning. Yet I was easily successful in the afternoon, long before the beginning of twilight. I will cite here several of my experiments. One morning I placed a Sphinx euphorbiz, which had begun to fly at 9 o’clock on the previous evening, on the window cur- tain, where it remained quietly. At 2:45 I returned it to its glass box, which stood in a dark corner and into which light fell only through a narrow slit. An hour went by, but the animal did not leave its place. It then moved to the light side of the box, without flying. I carried the animal back to the window, where it remained quietly. After twenty min- utes I returned it again to the dark box. Half an hour later, at half-past 4, it finally began to fly. The next day I allowed it to remain at rest near the win- dow, and it did not begin to fly until 9 Pp. M. at well-advanced twilight. On the following day I kept it in the dark box, and at half-past 3 in the afternoon it had already begun to fly. At noon on the succeeding day a heavy storm came up and it grew quite dark. The moth, which until then had remained quietly at the window, began to fly. I have had the same experience with other examples of this species. These facts seem to indicate that it is possible to influence the time of waking of Sphinx euphorbie by diminishing the intensity of the light, but only when they would soon wake up without artificial interference. Digitized by Microsoft® 42 STUDIES IN GENERAL PHYSIOLOGY The day butterflies are positively heliotropic like the night butterflies. The only striking feature is that in certain day butterflies the intensity of the light must be very great to bring about heliotropic movements. Specimens of Papilio machaon (which I had raised) remained at rest during the day at a window where they were exposed to the diffuse day- light and could be carried around on the finger; as soon, however, as they were brought into direct sunlight, they flew toward the window in the direction of the rays of light, and this with such force that they dropped down as if stunned. In direct sunlight they pressed themselves closely against the window pane. In diffuse daylight the animals, if they moved at all, crept toward the source of light; but in direct sunlight they flew toward it. My attempts to attract Papilio machaon by the weak light of a kerosene lamp were unsuc- cessful. I will add at this point my general observations on the caterpillars of butterflies. I have not found these periodic variations in heliotropic irritability in most caterpillars, not even those of Sphinx euphorbiae. The caterpillars which I studied reacted to light at all times of the day and night. The caterpillars agree, however, with the day and night butterflies in so far as they are all, without exception, positively heliotropic. This positive heliotropism is most marked in the cater- pillar of the willow-borer, which lives in the stems of the willow where it is not at all exposed to light. Such cases are also known in plants. Roots, for instance, are helio- tropically irritable, and yet, as Sachs points out, under nor- mal conditions their heliotropism is of no use to them. They can certainly not have acquired it through natural selection. According to the Darwinian theory, we would expect that the caterpillars of willow-borers should be nega- tively heliotropic, or at least indifferent to light. But the Digitized by Microsoft® HELIOTROPISM OF ANIMALS 43 behavior of an animal is merely the resultant of all its forms of irritability, and so it may happen that an animal is positively heliotropic even when it has no opportunity to make use of it. The larve of many saw-flies behave just as the caterpillars of Lepidoptera. I have made observations on the larve of Nematus ventricosus, which are exactly like those on Porthesia chrysorrhcea, which have been described. I have not yet succeeded in demonstrating a_heliotropic reaction to diffuse light in the indigenous pupe. Wilhelm Miller, however, has observed effects of light in South American species." The pupx can move at three joints. Only a lateral movement to the right and left is possible in some of the species; in other species only a dorsal move- ment of the body is possible; in a third species of pups a combination of both kinds of movements is possible. Miller observed that all three classes of movements can be brought about under the influence of light. He found that some pup turned not only away from the light, but also toward it. He also found that when the animals had been exposed to the dark for some time, they “needed some time to become susceptible again to the influence of light.” In interpreting the phenomena Miller follows the Darwinian idea, so that the thought never occurs to him that he might be dealing with phenomena similar to the heliotropic phenomena of plants. The negative geotropism of the Lepidoptera.— The movements of very young or recently hatched animals have for the most part been misunderstood, because they have always been considered a function of mysterious “instincts” of the animals, while the direction of their motions is in reality determined by definite external forces, The same cause which prescribes the course of a falling stone or deter- mines the orbits of planets, namely gravitation, determines 1 MULLER, Zoologische Jahrbicher, Vol. I (1886), pp. 568 ff. Digitized by Microsoft® 44 STUDIES IN GENERAL PHYSIOLOGY also the path which a butterfly follows that has just emerged from the pupa case. The geotropic irritability is at that time especially strong; the newly hatched animals remain restless, and are compelled to run about until they come to a vertical wall, on which they can put the longitudinal axes of their bodies vertically, with their heads upward. Here they remain quietly until their wings are unfolded. The powerful mani- festation of negative geotropism at the time of hatching is no isolated phenomenon in insects. In summer we find great numbers of the ecdyses of the larve of Ephemeride on the banks of streams. They are found on blades of grass or steep banks, with their longitudinal axes usually vertical and the head upward. That gravity, and not light alone, plays the chief réle here is shown by the fact that I have found the ecdyses in the same position under bridges where no light could strike them from above. This observation on the larvee of Ephemeride makes it impossible for us to accept the idea that the “purpose” of the orientation of the freshly hatched imago of a butterfly is that the wings may unfold; for negative geotropism appears in the larvee of Ephemeride at a time when no wings are present. The caterpillars of butterflies are also negatively geotropic like the freshly hatched moths, even though not so markedly. Immediately after hatching geo- tropism is much stronger in the imago of the butterfly than heliotropism—a phenomenon rarely observed in the animal kingdom. If a freshly hatched imago is on a vertical wall, it does not change its orientation toward the center of gravity even when the direction, refrangibility, or intensity of the light is changed. What is true of the heliotropism of Lepidoptera, that it is most marked during certain periods of their existence, holds good also for their geotropism. Amphipyra is ener- getically negatively geotropic immediately after moulting. Digitized by Microsoft® HELIOTROPISM OF ANIMALS 45 Several days later the animals assume every possible position with reference to the vertical. They prefer to remain on vertical walls, yet they will creep just as readily into hori- zontal folds and crevices. VII. THE POSITIVE HELIOTROPISM OF PLANT LICE Anyone closely studying a rose covered with wingless plant lice will notice that they are arranged in a definite way on the plant. On a vertical stem they rest with the head downward; on the leaves they are usually found on the underside, mostly on the principal veins. Here one also notices a certain regularity in their orientation, in so far as the animals on the principal vein turn their oral poles toward the stem, and their aboral poles toward the point of the leaf. The orientation of the animals seems therefore to be controlled by the structure of the plant, and not directly by external forces. But the plant lice do not behave on all plants as on the rose. On a palm, for example, I found no such definite orientation of the animals toward the plant, even though in this case also they show a preference for the lower surfaces of the leaves. Yet it might seem reasonable to suppose that light or gravity compels the plant lice to seek the lower surfaces of the leaves. I twisted several leaves of Cineraria, the dorsal sides of which were covered with plant lice, so that the dorsal sides were directed upward and toward the window, and fixed the leaves in this position. I watched the animals for two days and found by actual count that the animals remained: at rest. I repeated the same experiment on the plant lice of palm leaves, but also with negative results. My experiments on the orientation of new-born wingless plant lice were practically negative when I removed them from the plant and placed them in a glass vessel. Yet in Digitized by Microsoft® 46 STUDIES IN GENERAL PHYSIOLOGY the older wingless animals I could notice an inclination to move toward the source of light. When their wings had sprouted, however, the orientation of the plant lice was extraordinarily definite. In this state they are perhaps the most suitable animals we have for demonstrating the phenomena of heliotropism. Not all species are equally irritable ; Cineraria afforded me the best specimens. I have never found a species of plant louse which was not definitely positively heliotropic. I kept the plants near a closed window. The animals were attracted by the sun to the window, where they crept upward. When the animals are lightly touched with the point of a pen, they fall down a second or two later. If a glass vessel is held under them, a large number of these animals can be collected in an unin- jured condition in a short time. I found it much better to work with such animals as have already flown from the plant, than to collect the winged animals from the plant itself. To obtain the winged plant lice in great numbers it is necessary only to allow a plant which is covered with them to dry out gradually. Under such conditions the wings grow out very rapidly. All the experiments which were made with Porthesia chrysorrhoea can be repeated with exactly similar results on winged plant lice contained in a test-tube. As in the heliotropism of caterpillars, the heliotropism of plant lice is determined chiefly by the more refrangible rays, which compel the animals to move in the direction of the rays toward the source of light. If we place the test-tube containing the animals on a horizontal table, they always move toward the source of light, whether this be lamplight, diffuse daylight, or direct sunlight. The orientation occurs the more rapidly the more intense the light. If the intensity of the light is constant, the plant lice, like the caterpillars of Porthesia chrysorrhcea, are compelled to remain perma- Digitized by Microsoft® HELIOTROPISM OF ANIMALS 47 nently on the side of the test-tube which is turned toward the source of light. If direct sunlight comes through the window, and the tube containing the animals is so placed that one-half lies in the direct sunlight, while the other half is in diffuse day- light, and if the latter half is nearer the plane of the win- dow than the former, the animals will move to the window side of the vessel, like the caterpillars of Porthesia chry- sorrhcea; they leave the direct sunlight and move into dif- fuse daylight in order to follow as nearly as possible the direction of the rays. The result is the same when the dif- fuse daylight first passes through dark-blue glass. The animals are compelled to go to the window side of the tube under all conditions, no matter whether the test-tube is covered entirely or only in part by the blue glass, or whether the blue glass is placed over the window or the room side of the tube. The less refrangible rays which pass through deep-red glass are not very effective. In con- sequence, if the test-tube is entirely covered with red glass, the animals, if not very sensitive, distribute themselves evenly over the whole test-tube, just as in the dark; or, if more sensitive, they collect after a long time on the window side of the test-tube. But even then they do not quite behave as under blue glass. While under blue glass they collect in a very small area on the window side of the tube, under red glass they occupy a much larger area. If only a part of the test-tube is covered with red glass, the animals collect at the window side of the uncovered portion of the test-tube, as the less refrangible rays have only a minimal effect. When I placed a test-tube containing highly sen- sitive plant lice on a horizontal table perpendicular to the plane of the window, and covered the window side of the test-tube with a bright-red glass, the animals collected at the boundary between the uncovered and the covered part of the Digitized by Microsoft® 48 STUDIES IN GENERAL PHYSIOLOGY tube when diffuse daylight entered through the window. The more refrangible rays reflected from the walls of the room were more effective than the rays from the window which had passed through the light-red glass. This pre- vented the animals from going under the red glass. But when I used direct sunlight, the animals moved under the red glass to the window side of the vessel and remained there. When the animals were collected at the room side of the test-tube lying horizontally on a table and with its lon- gitudinal axis perpendicular to the plane of the window, and an opaque cover was placed over the room side of the tube, while a dark-red glass was placed over the rest of the tube, the animals went under the red glass and gradually collected there on the window side of the tube. But when I placed the opaque cover over the window side and the red glass over the room side of the tube, and the animals were under the opaque cover at the beginning of the experiment, they did not collect under the red glass. The rays reflected from the wall of the room had lost their directing power in filtering through the red glass. The experiments were made in the diffuse light of a dark day. On a bright day the animals moved to the room side of the tube under the red glass. The rays which pass through red glass have therefore the same effect, only they are weaker than the rays which pass through blue glass. I have already mentioned the fact that the day Lepi- doptera begin to fly as soon as direct sunlight falls upon them, while in diffuse light their heliotropic movements con- sist chiefly in creeping. The same difference in the effects of different intensities of light can be easily demonstrated in winged plant lice. In diffuse light of low intensity they move forward by creeping; when brought into the sun they Hy. To obtain a measure of the difference in the activity of Digitized by Microsoft® HELIOTROPISM OF ANIMALS 49 rays of different intensities and refrangibilities, I measured the time it required for the animals in a test-tube to pass a line scratched in the glass, when moving under the influence of light from the room side of the tube to the window. In these experiments I used some sluggish winged plant lice which I had taken from a plum tree. As a rule, the heliotropic movements of plant lice took place much more quickly than in the experiment to be described here. The experiment was made in diffuse light on August 8, 1888. At the beginning of the experiment all of the animals were on the room side. The animals passed the marks as follows: In the first minute 11 animals In the second minute 17 In the third minute 19 =“ In the fourth minute 21 . In the fifth minute 10 - In the sixth minute 12 Ke In the seventh minute 13 ts Only three animals had at this time not yet crossed the line. Several minutes later, at 9:20 o’clock, when the sun was coming through a fleecy white cloud, I made the following experiment in direct sunlight with the same animals. At the beginning of the experiment the animals were again on the room side of the tube. The animals this time passed the mark as follows: In the first minute 31 animals In the second minute 36 & In the third minute 23 8 Half a minute later the last sixteen animals had also passed the mark. The velocity of the movement was twice as great in direct sunlight as in ordinary daylight. These experi- ments were repeated and gave practically the same results. At 10:17 I placed the animals under a dark-blue glass. Digitized by Microsoft® 50 STUDIES IN GENERAL PHYSIOLOGY This time it took ten minutes for the animals to orient them- selves—a longer time, therefore, than in white light. At 10:29 I covered the test-tube with red glass, and since I knew that in diffuse light the heliotropic movements take place only very slowly under red glass, I brought the animals at once into direct sunlight. It required seventeen minutes before the majority of the animals had passed to the window side of the mark. In diffuse light it required an hour for orientation to take place under red glass; in a new experiment it required twelve minutes under blue glass. I noticed no periodic change in irritability in plant lice such as that observed in Lepidoptera, but I did notice a decrease in heliotropic irritability, a kind of rigor when the animals have been left undisturbed in the dark for some time. If the test-tube remained undisturbed, the animals remained permanently on the side nearest the window. When I very carefully turned the test-tube through an angle of 180° in the daytime, the animals again moved toward the window, even when they had been left undisturbed for hours. When, however, I kept the animals quietly in the dark, and after some hours carefully placed the tube near a lamp, the animals did not move from the position which they had maintained through the day. They seemed to be asleep. But when I shook the tube so that the animals began to move, they promptly oriented themselves toward the light as often as I turned the tube around. I found that winged plant lice are negatively geotropic as well as positively heliotropic, as is the case in the larve of Chrysorrhoea. If the animals in the test-tube were very vigorous, a change in the position of the tube with reference to the vertical brought about a change in the orientation of the animals toward the center of the earth; they traveled upward at as small an angle as possible with the vertical, and Digitized by Microsoft® HELIOTROPISM OF ANIMALS 51 collected at the highest point in the test-tube. This experi- ment must, of course, be made in a dark room. When the animals are first brought into the dark, the experiment can be repeated many times with exactly the same result; every change in the position of the test-tube with reference to the vertical compels the animals to creep upward and to collect at the highest point in the tube. When, however, the ani- mals were kept permanently in the dark, the reaction ceased soon, and the animals remained motionless, no matter how often the position of the test-tube was reversed. The ani- mals were in a sort of rigor. When they were placed on an inclined or vertical plane, they moved upward. Geotropic orientation occurred as soon as the plane made an angle of 30° with the horizontal; the geotropic movements were the more certain and precise the nearer the plane approached the vertical. When light fell on the animals at the same time, their orientation was determined by the resultant of the direction of the rays of light and gravitation, in which, how- ever, the light was the stronger force even at a great distance from the window. The winged animals behave toward a source of heat in the same manner as the caterpillars of Porthesia chrysorrhca. When I brought the animals in an opaque vessel into a room having a temperature of 18° and placed them near a stove, they left the side of the vessel which was turned toward the stove, as soon as its temperature increased a few degrees. At a temperature of 9° the animals were so sluggish that a definite reaction to light or gravity did not take place. A temperature of 20-24° is the most suitable for the experi- ments. When I surrounded one-half of the vessel with a water-bag having a temperature of 20°, the other half with one having a temperature of 10°5 the animals moved, under the influence of light, from the warmer into the cooler area. But they did not move far into the latter, as their movements Digitized by Microsoft® 52 STUDIES IN GENERAL PHYSIOLOGY soon ceased. Under the influence of light, the animals also moved from a region having a temperature of 12° to one having a temperature of 24°. VIII. THE CONNECTION BETWEEN HELIOTROPISM AND SEXU- ALITY IN ANTS At the time of sexual maturity the male and female ants fly from the nest on a warm day to pair in the air. This “nuptial flight’ is, as shown by the following observations, determined by a very pronounced positive heliotropism, which appears especially at the period of sexual maturity. I discovered a nest of brown garden ants in the wall of a house which was struck late in the afternoon by direct sun- light. In August, 1888, I observed that on warm days in the afternoon, as soon as the sun struck the wall, at about 5 o'clock, the winged ants came out in swarms and then flew away in the direction of the rays of sunlight. I procured a large number of winged ants from such a swarm and studied their behavior toward light. These animals were energeti- cally positively heliotropic, and behaved in all respects like the caterpillars of Porthesia chrysorrheea. When I put the winged ants into a test-tube and placed this with the longitudinal axis perpendicular to the plane of the window, the animals moved to the window side as often as the tube was turned around. The velocity of the helio- tropic movements was greater in these animals than in any others that I have studied. When the tube was not disturbed the animals remained on the window side nearest the win- dow. When the longitudinal axis of the test-tube lay par- allel to the plane of the window, the animals distributed themselves evenly over the whole length of the tube. When one-half of the tube was in direct sunlight, while the other half was in diffuse daylight, but nearer the window, the ani- mals collected in the window side of the tube, they went from Digitized by Microsoft® HELIOTROPISM OF ANIMALS 53 the direct sunlight into theshade. The direction of the rays, and not the distribution of the intensity of the light, tn the test-tube, therefore, determines the direction of the pro- gressive movements. The blue rays were pre-eminently effective. When the test-tube was covered with blue glass, either entirely or in part, the orientation was changed in no way. When the tube was entirely covered with red glass, the movements -oecurred more slowly. The animals finally collected on the window side, but it took a long time. When the tube lay with the longitudinal axis perpendicular to the window, and the portion nearest the window was covered with red glass, the animals collected at the boundary between the uncovered and covered parts. Diffuse daylight affected the animals just like sunlight. These facts may suffice to show that at the time of the nuptial flight the winged ants are energetically positively heliotropic. Yet I found that up to the time of the nuptial flight, light had practically no effect on winged ants which were taken from the same nest. Animals which I collected after the nuptial flight also did not react very distinctly to light. If heliotropism was still present at all, it was obscured by other forms of irritability, particularly stereotropism. The nuptial flight of the ants of this nest always took place at about 5 o’clock in the afternoon, when the sun’s rays fell upon the nest. That it was the latter condition, and not the time of day, which determined the period of flight is shown by the fact that in other nests, which were reached by the sunlight earlier in the day, the flights took place earlier. Usually the flight occurs at about noon, when the sun’s rays strike the earth perpendicularly and the tem- perature is relatively high. Both the males and the females which I collected from the swarm which had left the nest Digitized by Microsoft® 54 STUDIES IN GENERAL PHYSIOLOGY late in the afternoon escaped through the window on the next day at any time that I freed them. The scent of the females therefore does not determine the nuptial flight of the males, and vice versa; after sunset the ants no longer flew away when liberated. I have already shown that direct sunlight or intense dif- fuse daylight calls forth flight movements in plant lice and day Lepidoptera. This also occurs in winged ants. In dif- fuse daylight the male and female ants move toward the source of light only by using their legs; in direct sunlight, however, they fly. Sunlight, therefore, causes flight movements in ants at the time of sexual maturity, and this fact determines the nuptial flight. Immediately after copulation another form of irrita- bility becomes more prominent’ which compels the ants to to crowd into crevices (to “found a new nest”). The connection between sexuality and heliotropism in ants is shown still further by the fact that at the time of the nup- tial flight no heliotropism can be demonstrated in the workers. Workers taken from the same nest as the other ants when placed in a test-tube moved about irregularly in it, and finally came to rest on the stopper, no matter in what position I placed the tube with reference to the window. I then placed several winged ants which reacted energetically toward light in the same tube with the workers. The workers apparently became now also positively heliotropic, that is to say, they moved with the winged ants to the window side of the tube whenever it was reversed. This lasted, however, only some ten minutes, when the workers settled again permanently on the stopper and were no longer affected by the light while the winged ants reacted to the light just as before. The observations of Lubbock seem to indicate that helio- tropism may be present also in the workers at certain periods 1Stereotropism. [1903] Digitized by Microsoft® HELIOTROPISM OF ANIMALS 55 in their existence. In the experiments of Lubbock the workers contained in the nest not only collected under red glass, but also carried their larve there. The animals are therefore negatively heliotropic.’ All these facts, however, do not yet exhaust the connec- tion between sexuality and heliotropic irritability. The heliotropism of the male and female ants is also different, inasmuch as it requires more intense light to cause helio- tropic movements in females than in males. In isolating the males and females of the same swarm I noticed that the females had ceased to execute heliotropic movements before it seemed as if twilight had really begun. The males how- ever still collected on the window side of the tube long after sunset. Experiments with colored glasses succeeded in males when the light was so faint that I had difficulty in dis- tinguishing the color of the glasses. On dark, cloudy days females showed no heliotropic reactions toward the window, while the males did. It harmonizes with this observation that on cloudy afternoons I saw occasionally winged males leave the nest, but no females. As soon as the intensity of the light had become so small that heliotropic phenomena were no longer produced, another form of irritability appeared in the winged ants, especially in the females, namely, stereotropism. The animals then crowded into all crevices. I placed the animals in a dark box, and laid a small, folded piece of velvet into one corner of it. After a few moments they had crept into the folds of the velvet. With the males it took a much longer time than with the females. This irritability, however, did not appear as long as the light was sufficiently intense to call forth heliotropic phenomena. When exposed to light, the animals crept neither under the piece of velvet nor into crevices. It is very probable that a similar difference in heliotropic irri- 1 The observations recorded in Lubbock’s paper admit another possibility. [1903] Digitized by Microsoft® 56 STUDIES IN GENERAL PHYSIOLOGY tability exists in the two sexes of the Lepidoptera. Réaumur states that in the main only males fly into the candle flame. From this fact, which is correct, it follows that it must require a more intense light to cause the females to execute heliotropic movements than is necessary for the males. Both male and female moths are attracted by sources of light which are stronger than the candle flame, for instance, the electric arc light. It is a well-known fact that the females fly less than the males. It is imaginable that this is due to the fact that the females are less irritable toward the light than the males. The difference in the irritability of male and female ants toward light brings up the question as to whether the differ- ence in the development of the sense organs, particularly the eyes, which is often observed in males and females of the same species, is connected with this difference in irrita- bility. The males of ants have larger eyes than the females. But the cause of the difference in sensitiveness may lie deeper, as is, for example, indicated by the following obser- vation made by Semper: “In all species of the cave beetle Macheerites only the females are blind, while the males have well-developed eyes ; notwithstanding this fact they always live together.”’ Eyes therefore develop more easily in males than in females even in the dark. It might be worth while to determine whether in these cave-dwellers the males are also heliotropically more sensitive than the females. IX. THE NEGATIVE HELIOTROPISM AND OTHER FORMS OF IRRITABILITY OF THE LARVE OF MUSCA VOMITORIA The phenomena of irritability in negatively heliotropic animals obey the same laws as those in positively heliotropic animals ; with this difference, however, that negatively helio- tropic animals turn their aboral poles toward the source of light instead of their oral poles, and that in consequence the 15EmPER, Die natirlichen Existenzbedingungen der Thiere, Vol. I, p. 101. Digitized by Microsoft® HELIOTROPISM OF ANIMALS 57 direction of their progressive movements under the influence of the rays of light is away from the source of light. My description of negative heliotropism need therefore be but brief. I have chosen as an example of negatively heliotropic animals the larva of Musca vomitoria, which are addi- tionally interesting in that they are completely blind. Helio- tropism in animals is therefore a characteristic of their protoplasm, and not a specific characteristic of their eyes; just as in plants, which have no eyes. In order to study the negative heliotropism of Musca larvee it is best to take the almost fully grown larve fresh from the cadaver on which they were reared. When the light, which may be either diffuse daylight or direct sun- light according to the sensitiveness of the animals, is of the proper intensity, the directing influence of the rays of light can be demonstrated more beautifully in the larve of the fly than in any other animal. I placed a number of these animals on a horizontal board and exposed them to sunlight. This was at about 4 o’clock in the afternoon, when the rays of light fell obliquely through the window. I shut out that part of the rays which came through the window from above by means of blinds. As soon as the animals came into the sunlight, they were oriented with their oral poles toward the room, and their aboral poles toward the window. They crept with mathematical precision in the direction of the rays. When a shadow was thrown on the board by a penholder, it could be noticed that the animals moved away from the light in a direction exactly parallel to the edge of the shadow. The directing force of the rays was so strong that the animals crept closely along the edge of the shadow without crossing it. They acted as though they were impaled on the ray of light which passed through their median plane. When I turned the board around, the animals immediately turned about also, and again placed Digitized by Microsoft® 58 STUDIES IN GENERAL PHYSIOLOGY their median planes in the direction of the rays. (That this was due to the effect of the light, and not a compensatory movement that might have been produced by the rapid turning of the board is shown by the fact that compensatory movements do not exist in Musca larve.) I was able to show that fly larvee are compelled to move from less intense light into more intense light under the influence of the rays of light, just as it could be shown that positively heliotropic animals do not go from dark places to light ones, but follow the direction of the rays, even when by so doing they move from a region of greater intensity of light to one of less. I put the almost fully grown larvee into a test- tube and placed it horizontally on the table, with its longitu- dinal axis perpendicular to the plane of the window. The sun’s rays made a small angle with the window. By means of a screen I arranged the test-tube so that only diffuse light fell through the window upon the half turned toward the window, while direct sunlight fell upon the half turned toward the room. At the beginning of the experiment the animals were all on the window side of the test-tube. They immediately moved from the shaded part into the direct sunlight on the room side, and remained there. Incidentally I was able to observe that the light stimuli which strike the oral pole of these completely blind animals are most important in the orientation of the animals toward light. When the animals crossed the boundary from diffuse light into direct sunlight, the reaction caused by the increase in the intensity of the light did not take place until a half or a third of the body of the animal was in the sunlight (because in all phenomena of stimulation some time elapses between the application of the stimulus and the reaction to it). The animal checked its movement and turned its head through an angle of 90-130° from side to side. If in so doing the head again came into the shade, the animal Digitized by Microsoft® HELIOTROPISM OF ANIMALS 59 returned into the shade; but if this did not happen, as was more usually the case, the animal continued its movement into the sunlight. The animals did not always check their movements in passing from a shaded area into the sunlight. Often they moved without delay from the shade into the sunlight. The following observation shows that the rays of light which strike the head mainly determine the orientation : When I placed a fully grown animal on a board, and pushed the board from the shade into the sun, so that only the head of the animal was struck by sunlight, the larva immediately placed its median plane in the direction of the sun’s rays. When, however, I put only the aboral pole into the sunlight, this orientation did not occur. Animals from which the first few segments of the oral pole had been amputated no longer oriented themselves toward the light. Yet little weight is to be given to vivisection experiments, which are followed only by an inhibition of the effects of a stimulus. When I allowed the sun’s rays to fall on the plane of the board perpendicularly, the animals moved over it in all directions. As in this case the animals could not follow the direction of the rays of light, it had no other influence upon them than to increase their restlessness, and no uniform orientation resulted. It could be shown very beautifully in these full-grown larvee that essentially only the more refrangible rays are con- cerned in exercising a directing influence upon these animals. I placed a large number of fully grown larve on the middle of a horizontal board in a darkened room, and exposed them to the sun’s rays which made but a small angle with the horizon. Within ten to twenty seconds every animal had placed its median plane in the direction of the rays of light, and moved exactly parallel to the shadow of a vertical object which had been thrown upon the board for comparison. I treated a new lot of animals in exactly the same way, but Digitized by Microsoft® 60 STUDIES IN GENERAL PHYSIOLOGY before exposing them to the sunlight I covered them with a box of dark-blue glass. Within ten to twenty seconds these animals had also placed their median planes sharply and precisely in the direction of the rays of light, in which direction they moved toward the room side. When I took a third lot of fresh animals and covered them with red glass, the orientation of the animals into the direction of the rays did not occur. They crept to the right and to the left, occa- sionally moving a short distance toward the source of light; but even after minutes under the red glass the precise orien- tation of the animals, which followed under the blue glass in a few seconds, did not occur. Under red glass the animals behaved toward direct sunlight just as they did under blue glass toward very weak daylight. That the rays which pass through red glass are not absolutely without effect seems to be shown by the fact that the animals avoided going to the window side, and that they finally collected at the room side of the board. The directing force of the red rays seems therefore to be limited to this, that the animals will not move for long distances toward the source of light. In con- sequence, the animals must collect ultimately on the room side of the vessel. In all the previous experiments the animals were on a plane board. When at the beginning of an experiment the animals were collected on the window side of a test-tube which lay horizontal and perpendicular to the plane of the window, in direct sunlight and under blue glass all the ani- mals turned their oral poles within ten seconds toward the room side of the tube. In about twenty seconds they migrated to the room end of the tube. When the same ani- mals were exposed in the same way to direct sunlight, but under red glass, they neither oriented themselves nor moved toward the room side of the tube during the next four minutes, even though they were very restless. Digitized by Microsoft® HELIOTROPISM OF ANIMALS 61 In this case, therefore, just as in the case of the positively heliotropic animals, it is chiefly the more refrangible rays which effect the orientation of the animals. The helio- tropic influence of the less refrangible rays, however, is much less in the eyeless fly larve than in any other ani- mals that I have studied. The animals moved in the direc- tion of the rays, even in diffuse light, at a distance of one meter from the window, but the less the intensity of the light, the more easily did other stimuli (such as contact stimuli) cause a deviation of the animals from the straight line. I have often repeated these experiments in the course of the last two years, and have each time obtained essentially similar results. The irritability of the animal is, however, not always the same; especially does its irritability vary during different periods of its life. I have, however, con- vinced myself that the larve are negatively heliotropic even immediately after being hatched, although they do not move as precisely in the direction of the rays as the fully grown larvee. I placed some fly eggs on smoked glass plates and allowed them to hatch. As the larve removed the soot in their path, they thus registered graphically the paths they took from the eggs. The glass plates lay on a horizontal table in a room lighted from one side only. The paths followed by the larve ran, almost without exception, toward the room side of the plates. In the few exceptions the path usually ran first toward the window, then bent, and went toward the room side of the plate. It was neither a mysterious force of nature nor an obscure “inherited instinct” which dictated the direction of the movements of these animals, but only the direction of the rays of light, Just as gravity determines the orientation of the Lepidoptera when they emerge from the chrysalis. When the diffuse daylight which struck the larvee came Digitized by Microsoft® 62 STUDIES IN GENERAL PHYSIOLOGY from two windows the planes of which were at an angle of 90° with each other, the paths taken by the larvee lay diago- nally between the two planes.’ When I placed the plate with the eggs in an absolutely dark room, the paths followed by the larve ran concentrically around the nest; the animals had scattered equally in all directions over the plate, but, contrary to the behavior of the animals in the light, which always moved as far as possible toward the room side, the circle in which the animals moved in the dark was very nar- row. They did not leave the glass plate. The constant intensity of the light acts, as in the case of the positively heliotropic animals, as a constant stimulus which causes the animals to move in one definite direction (either toward or away from the source of the light), until some other stimulus intervenes, which modifies or abolishes the effect of the light. In ny preliminary communication on animal heliotropism I mentioned an effect of light on fly larvee which I called a kind of anisotropy, and which I am at a loss how to in- clude under the other phenomena of heliotropism. The phenomenon under discussion appears only in intense light and in newly hatched or very young larve. The phenomenon consists in this, that the animals’ turn their ventral surfaces toward the source of light without placing their median plane in the direction of the rays. I have never seen this orientation in adult larve. When I put the animals into a test-tube placed with its longitudinal axis perpendicular to the window, and exposed them to the direct rays of sunlight or diffuse light close to the window, the animals left the lower side of the tube and moved to the upper. In this the animals, therefore, resembled positively heliotropic animals, and I might have believed that I was dealing with one of 1This experiment was recently published by an American physiologist as a new discovery to prove that I had overlooked the importance of the intensity of light! [1903] 2 When kept in a test-tube. [1903] Digitized by Microsoft® HELIOTROPISM OF ANIMALS 63 those cases occasionally observed in plants where in light of great intensity the heliotropism of an organ is the opposite of that in light of less intensity. A closer examination, however, showed this not to be the case. When the test- tube lay perpendicular to the plane of the window, positively heliotropic animals contained in it moved, as we have seen, not only to the upper, but also to the window side of the test- tube. This was not the case with the newly hatched fly larve. They all turned their ventral surfaces toward the source of light, but otherwise moved about irregularly. I placed the animals in a test-tube which was covered with black paper, except for a small slit, and let direct sunlight enter the tube only through the slit. The animals which were on the lower side of the tube left it as soon as the light struck their backs, and crept upward; but no animal which was sheltered from the light was attracted to the upper, lighted side of the tube, as was the case under similar con- ditions in the positively heliotropic caterpillars of Chrysor- thea. When I held the glass vertically, more animals collected on the window side, but they did not all creep up- ward, as did the positively heliotropic animals. When I placed the animals on the outside of a test-tube, they did not move upward, but collected for the most part on the under side of the tube. This experiment was not very decisive, however, as the animals easily fell off the tube. These facts can be interpreted in no other way than that the intense light compels the fly larve to turn their ventral surfaces toward the source of light, in which condition they are indifferent to the orientation of their median planes toward the rays of light. The ventral position is assumed only when the animals are exposed to light. With this, however, the striking features of the movements of orien- tation in fly larve are by no means exhausted. While the movements of orientation in all the other animals go on Digitized by Microsoft® 64 STUDIES IN GENERAL PHYSIOLOGY under blue glass just as rapidly and in the same way as in mixed daylight—since in mixed daylight it is chiefly the more refrangible rays which are heliotropically effect- ive—the ventral orientation of fly larvee which has just been described occurs neither under blue nor under red glass. In direct sunlight it took one to one and a half minutes before the animals were densely gathered on the upper side of the horizontally lying test-tube. Not one of these animals moved to the upper side of the tube in less than twenty-five minutes under red glass, or in less than five minutes under blue glass. The ventral orientation of the Musca larve toward a source of light can be observed most distinctly in freshly hatched larvee. As the animal grows larger, the phenomenon becomes less marked. The lump of eggs laid by a fly was distributed among three tubes. In all three tubes the animals immediately after hatching oriented themselves ventrally toward the diffuse light. I then fed meat to the animals in one tube and left the animals in the other two tubes unfed. On the next day the unfed animals were oriented ventrally toward the daylight, while this was not the case in the rapidly growing larvee which had been fed. I have ob- tained the same result by feeding the larve of one lot of eggs with fat, while another lot was given lean meat. The latter grew more rapidly than the former. While those fed on fat were oriented ventrally in diffuse daylight, direct sun- light was necessary to bring about this effect in those fed on meat. I might have doubted that this was the effect of light, had I not been able to prove that with a decrease in the in- tensity of the light the phenomenon becomes less distinct, and finally disappears entirely. I do not think that the ventral orientation could have been the effect of heat, as the animals move away from a non-luminous source of heat, as Digitized by Microsoft® ~ HELIOTROPISM OF ANIMALS 65 we shall see presently. Because of the preponderance of this ventral orientation, it is no easy matter to demonstrate the negative heliotropism of the young larve in a test-tube; they assume the ventral orientation, and no longer trouble themselves about the direction of the rays of light. The orientation of the larvee of Musca toward a source of heat,—If a Musca larva in its movements comes to a spot where the temperature is only one degree higher than in the sur- rounding area, it stops and turns its head laterally. If in so doing its head encounters a spot with a lower temperature, it turns thither and continues to move in this direction. One can easily convince oneself of this by laying the tip of a finger on a spot on the outside of a test-tube containing the larve. The increase of temperature of the spot touched can be ascertained by a sensitive and finely graduated thermom- eter. As soon as the animal comes to the spot touched by the finger, it turns its head. If it does not turn far enough to touch:a cooler spot, it continues in the old direction to the region of higher temperature. According to this, the stimuli which reach the oral pole determine the orientation of the animal toward a source of heat also, just as in the case of light. If the experimenter puts a test-tube containing a large number of Musca larve into his pocket, where no light reaches them, the animals collect in a few minutes densely on that side of the tube which is turned away from his body. The same thing happens when the tube is exposed to the rays of a non-luminous source of heat. Tf one-half of a tube is surrounded by a water jacket of a higher temperature, and the other half by a water jacket of room temperature, the animals in the warmer part become restless or perish ; they are not oriented, however, and consc- quently cannot save themselves by moving into that portion of the tube having a lower temperature. In these experiments the animals were contained in a Digitized by Microsoft® 66 STUDIES IN GENERAL PHYSIOLOGY long test-tube a (Fig. 4). The upper half was surrounded by a wide hollow cylinder b, the bottom of which was com- posed of the cork stopper ¢, into which a fitted water-tight. The lower half of @ extended into the hollow cylinder d. 6 and d were filled with water to a given height. When the temperature of the water in the cylinder b was 84°, while that of the water in d was 18°, the animals in the upper part of the tube became very restless, but did not creep into the cooler part of the tube. Nor were the animals oriented when the temperature in the lower part of the : cylinder was higher than in the upper. Such FIG. 4 an orienting effect of temperature was observed only when an animal came to the boundary between the warmer and the cooler zones at c. In such a case the animal moved into the zone having the lower temperature, but not into the other. By means of diffuse daylight, however, it was an easy matter to drive the animals from a place of lower tempera- ture to one of higher. This is possible because the light orients the animals and dictates to them sharply the direc- tion of their progressive movement, while the same is done by a source of heat only to a slight extent. It was therefore possible to drive the animals from diffuse light into direct sunlight, notwithstanding the difference in temperature. At low temperatures, even + 10°, it is scarcely possible to demonstrate the heliotropism of fly larve. Heliotropic experiments in these animals succeeded best at a tempera- ture of 20-25 °. The orientation of Musca larve toward chemical stimuli. —If on a summer day a piece of meat is set in the open, blow flies collect on it in great numbers and deposit their eggs. There can be no doubt that a chemical stimulus attracts the animals and causes them to lay their eggs. Digitized by Microsoft® HELIOTROPISM OF ANIMALS 67 Decaying meat has the same attractive effect on the larva of flies. If such animals are in a test-tube containing decaying meat, and the tightly fitting cork is loosened a little, the animals which were crawling between the meat and the open end of the tube turn and go back toward the meat. I mois- tened a small area on a plate by rubbing it with decaying meat. I placed some half-grown larvee which I had taken from the meat in the middle of this moist surface. They at first crept toward the room side of the plate, but turned when they came to the boundary of the surface smeared with the putrid meat, and remained within it. Not until half an hour later, when the spot had dried completely, did they leave it. When I merely moistened a spot on the plate with pure water, the larve did not remain on it. When I removed the animals from a cadaver and placed them on a glass plate, and brought a piece of decaying meat into their neighborhood, the animals crept toward it, even if in so doing they were obliged to move toward the window; this occurred, however, only when the animals were in the immediate neighborhood of the meat. When they were more than a centimeter and a half away from the meat, they were no longer attracted by it; they then followed the direc- tion of the rays of light and starved in the neighborhood of food. Animals which had not yet tasted food were also attracted by the decaying meat. Fat, even when foul, attracted the animals only slightly or not at all; this is very remarkable, as the female flies are also more readily attracted by meat than by fat. I often placed a piece of horse flesh and a piece of horse fat side by side in the sun. Ata time when the flesh was covered with eggs, the fat was almost free from them. It seems, therefore, as though the same chemical stimulus which attracts the larve causes the flies to deposit their eggs. Decaying cheese also attracted the larve, but ammonia and assafcetida were without effect. Some volatile Digitized by Microsoft® 68 STUDIES IN GENERAL PHYSIOLOGY substance must be formed in the decomposition of the pro- teids which attracts the Musca larve even from a distance. The contact-irritability of Musca larve.—tlit is a well- known fact that Musca larve are inclined to crowd into cracks and crevices in the earth, and it is astonishing through what small cracks the adult larve can slip. This irritability might impress a Darwinian as though the ani- mals wished to protect themselves from the light. That this contact-irritability is entirely independent of their helio- tropism is shown by the fact that these animals crowd them- selves under a completely transparent glass plate, even if by so doing they have to move toward the light. The animals retain this form of irritability even when put into a vessel of water, in which they soon die. I noticed this phenomenon in feeding tritons with fly larvee. Small stones lay on the bottom of the vessel, and the larvee crowded themselves under them as eagerly and as skilfully as if they had always lived under them. The perniciousness of this irritability in the case in question is apparent when we remember that it keeps the animals from reaching the sur- face of the water again, so that they are drowned. In these experiments I was struck by the fact that the animals, when placed under the surface of the water, do not swim upward and so avoid death, but swim downward. I cannot explain this fact. Under other conditions positive geotropism cannot be demonstrated in these animals. The positive heliotropism of flies at the time of sexual maturity.—The fly, which as a larva is negatively helio- tropic, is positively heliotropie in the state of sexual maturity. This reversal in the sense of heliotropism in changing to the adult state is not uncommon. Yet it is a striking fact that, while heliotropism is reversed, the orienta- tion toward chemical substances is the same in the female flies at sexual maturity as in the larval state. Digitized by Microsoft® HELIOTROPISM OF ANIMALS 69 Positive heliotropism can be demonstrated in flies by the same experiments as in plant lice; only it must be noticed that flies are provided with several more kinds of irrita- bility than plant lice, and that in consequence heliotropism may be obscured when other stimuli besides light come into play. In one experiment, for example, I observed that light, gravity, and heat were without effect [4 on the flies, because the animals always remained on the cork stopper in the test-tube. Some sub- stance was probably on the stopper that attracted the flies; for when I put the animals into a flask with a clean glass stopper, they reacted to light. I am indebted to Professor Ernst Mach for a beautiful observation on the influence of light on the orientation of the house fly: Several years ago I accidentally made an observation which I have never been able to follow further. While adjusting my rotat- ing polarization apparatus in a dark room, by the help of sunlight, whereby a bright quadratic picture a some 16 cm. across (Fig. 5) was rotated three or four times per second in a circle of a radius of 30 cm., a fly F (Fig. 6) happened to enter the bundle of rays LL, went through the L whole rotation as though stunned, and fell upon the table. I was able to repeat this 4 in| experiment twice. The fly, which was appar- Se ently sound, escaped while I was giving my attention to something else. FIG. 6 FIG. 5 In this case, then, the same effect was produced by rotat- ing the rays of light as by revolving the fly on a centrifugal machine." 1R&dl has recently come to the conclusion that the reactions of insects on a centrifugal machine are indeed caused by the light. If this is correct, they are identical with Mach’s observation. [1903] Digitized by Microsoft® 70 STUDIES IN GENERAL PHYSIOLOGY X. THE NEGATIVE HELIOTROPISM OF THE LARVE OF TENEBRIO MOLITOR The larve of a beetle Tenebrio molitor, which can easily be collected in large quantities, are also suitable animals upon which to demonstrate negative heliotropism. When such animals are placed on a plate, they creep to the room side of it; if the intensity of the light is sufficiently great, they remain there. If the plate be covered with dark-blue glass, the result of the experiment is the same. If the plate be covered with red glass, the animals move in the concave edge of the piate both toward the window and away from it; a definite orientation does not occur. Under red glass they behave just as in the dark; under blue glass, just as in the light. I covered one-half of a plate with blue glass and one-half with red glass, so that the boundary between them lay in the direction of the rays. The animals were distributed equally over the plate at the beginning of the experiment. All the animals in the blue half moved to the room side of the plate, but none in the opposite direction; in the red half they moved in all directions. The animals moved from the blue into the red, but never from the red into the blue. When I covered one-half of the plate with opaque cardboard, the other half with red glass, so that the boundary between them again coincided with the direction of the rays of light, the animals scattered in all directions in the two halves of the plate. After a long time, however, the greater number col- lected under the cardboard. The experiments which have been described were made in direct sunlight. If on a dark day the plate is some distance from the window and the light is not very intense, the ani- mals, which at the beginning of the experiment were in the middle of the plate, will gradually creep toward the room side; when, however, they reach the shallow groove in the Digitized by Microsoft® HELIOTROPISM OF ANIMALS 71 plate, they do not again leave it, and now creep toward the window also. The animals are forced to bring the surfaces of their bodies as much as possible in contact with other solid bodies. These phenomena are not altered when the plate is cov- ered with blue glass. If, however, it is covered with red glass, the animals, even when in the middle of the plate, move as frequently toward the window as toward the room side. So far as the stereotropism of these animals is con- cerned, it must be added that the animals collect in the con- cave edges of dark boxes. It might be supposed that the function of stereotropism is to protect the bodies of the animals from evaporation as far as possible. I covered one-half of the bottom of a box with a moist cloth and the other with a dry one, and, after putting fifty animals in each half of the box, I placed it in the dark. After two hours not a single animal was found in the moist half of the box. The animals flee from moisture and seek dry spots. Contact-irritability and negative heliot- ropism determine the habits of these animals, which live protected from the light, in flour. The negative heliotropism of the larve of June bugs.— The behavior of the larve of Melolontha vulgaris is quite similar to that of Tenebrio molitor. As they move for the most part while lying on their sides, their orientation takes place rather slowly ; nor do they follow in the direction of the rays of light as sharply as do the animals which have been described above. They flee from the light and move from the window to the room side of a vessel. The following experiment, which also serves to give an idea of the time required for experiments on these animals, shows that only the more refrangible rays are of chief importance in bringing about the heliotropic phenomena: At 10:40 o’clock I placed twenty-three larvee in the middle Digitized by Microsoft® 2 STUDIES IN GENERAL PHYSIOLOGY of a round plate covered with blue glass. The animals moved to the room side of the plate, tried to creep over the edge, and at 10:45 came to rest on the room side. I waited five minutes, and at 10:50 substituted red glass for the blue. The animals scattered equally over the whole plate, and at 11 nine animals were on the window side, the rest about uni- formly scattered over the whole plate. I then substituted blue glass for the red. At 11:07 all the animals were col- lected on the room side of the plate. At 11:10 I again covered them with red glass. The animals immediately began to creep over the plate in all directions. At 11:20 twelve animals were collected near the window, six in the middle, two on the side, and three on the room side of the plate. I kept the plate covered with red glass, and watched to see whether after a time the rays going through the red glass would not also bring about an orientation. No change occurred in the course of the next hour. Gradually, how- ever, more and more animals moved to the room side of the plate, and at 3:30 all but five animals were collected here. The animals, therefore, finally show a negative heliotropism under red glass also. The rays passing through red glass are therefore similar in their effects to those which go through blue glass, only they are not so effective. In this respect the behavior of these animals corresponds with that of plants. The larvee burrow into the ground. Negative heliotro- pism may co-operate here, but stereotropism is without doubt the chief factor concerned. The question arises whether it is not geotropism which causes the animals to bore into the ground, as in the case of roots. In order to determine this I made the following experiment: I filled a hollow cardboard cylinder, some 5 cm. in diameter, with earth. The cylinder was about 20 cm. high. I fastened the cylinder on a stand, with its longitudinal axis vertical, and brought it so near to a table that it Digitized by Microsoft® HELIOTROPISM OF ANIMALS 43 just touched two larvee lying upon it. I also placed two larvee on top of the cylinder. If the animals were negatively geotropic, the upper animals should have buried themselves more quickly than the lower. But the opposite was the case. After forty-five minutes the lower animals had burrowed upward so that they were completely out of sight; the upper were not buried until an hour later. There- fore, even though they may be negatively geotropic, for which I have as yet no proof, the contact-irritability of these animals determines that they shall burrow into the ground. XI. THE DISTRIBUTION OF HELIOTROPIC PHENOMENA IN THE ANIMAL KINGDOM The experiments which have thus far been described were carried out on insects. So far as experiments on representatives of the other divisions of the animal kingdom are concerned, I have con- firmed the identity of animal with plant heliotropism on crabs (Gammarus locusta, Cuma Rathkii), naked snails and worms (leeches, planarians, earth-worms and others). Experi- ments on infusoria are already sufficiently complete to show that Sachs’s laws of heliotropism also hold good for them.’ Investigations have not yet been made on Ccelenterates and Echinoderms; Trembley’s experiments on Hydra, how- ever, show that in their case also the relation is the same; at least it seems to me that Trembley’s experiments cannot be interpreted unless we assume that the progressive movements of Hydra are determined by the direction of the rays of light. I used the following method with aquatic animals: To prove that the direction of the rays determines the direction of the progressive movement, I used a long, four-cornered glass box, one wall of which was made of a watch-glass. The 1See the papers of Strasburger, Engelmann, and Stahl cited in the introduction. Digitized by Microsoft® 74 STUDIES IN GENERAL PHYSIOLOGY convex side of the watch-glass was turned outward. When direct sunlight fell upon the glass, the rays were focused a few centimeters behind the glass wall. Notwithstanding this fact, the positively heliotropic animals moved in the direction of the rays from the room side of the glass box through the focal point to the front of the box, although the intensity of the light was the greatest in the focus. This could be shown very beautifully in some tiny, positively heliotropic worms I found in the brackish water at Kiel, but whose identity unfortunately I failed to determine. Positive heliotropism is encountered more often in the plant kingdom than negative heliotropism. It is worth while to mention the fact that positive heliotropism appears to exist in more species in the animal kingdom also than does negative heliotropism. All caterpillars and Lepidoptera, whether they fly by day or night, can, according to my observations, be con- sidered positively heliotropic. Thus far I have tried in vain to find negatively heliotropic Lepidoptera or caterpillars. The great majority of the other winged insects are also positively heliotropic. We also encounter positive heliotropism in animals which live in water, and even in mud, and which therefore can never profit by light. I was much interested in some obser- vations I made in this direction on a small Crustacean (Cuma Rathkii) which lives on the bottom of the bay of Kiel. The animal can be fished out of the mud in which it buries itself only with a dragnet. Notwithstanding this fact, the animal is strongly positively heliotropic. When I kept these small crabs in a glass vessel and allowed light to fall upon them from one side only, the moving animals collected at the side of the vessel nearest the light. The resting animals were oriented, and turned their oral poles toward the source of light and their median planes in the direction of the rays. Digitized by Microsoft® HELIOTROPISM OF ANIMALS 15 When the source of light or the vessel was carefully turned about, the animals changed their orientation until their median planes were again in the direction of the rays of light. The directing force of the light exhibited itself here in the same manner as in the Euglenz in the experiments of Stahl. I next placed a small glass box filled with mud in the same vessel with the crabs. The animals did not scent the mud; at least not one of them moved into the box containing the mud. When I disturbed the animals (by touching them with a pencil), they first swam upward and then, if I did not disturb them further, slowly fell to the bottom. If an animal happened to fall upon the mud, it immediately became lively as soon as it touched the mud. It burrowed into it eagerly, after which it was impossible to get the animals to react to light. The other animals which fell to the glass bottom of the vessel remained inactive. Thus we see that contact with the mud had a greater effect than light; contact-irritability is more intense than heliotropism. It is in this way that it happens that the animal, besides being a poor swimmer, lives away from the light in spite of its positive heliotropism. XII. THE DEPENDENCE OF THE ORIENTATION UPON THE FORM OF THE BODY In the introduction I have called attention to the fact that the orientation of an animal toward light, like every phe- nomenon of irritability, is determined by two factors: first, external causes—in this case the light—and, second, inter- nal causes, namely, the structure of the animal. So far as the structure of the animals is concerned, we are dealing in this paper exclusively with animals whose bodies consist of two morphologically symmetrical halves, and which have morphologically different ventral and dorsal Digitized by Microsoft® 76 STUDIES IN GENERAL PHYSIOLOGY surfaces and morphologically different oral and aboral poles. We disregard all other detailed morphological peculiarities, because those mentioned suffice to explain the orienting movements of an animal, as they do for the movements of plants. The distribution of irritability on the surface of an animal corresponds to the above-mentioned morphologi- cal relations. Elements at the surface of the body sym- metrically situated with reference to the median plane have equal irritabilities. This condition compels the animal to orient itself toward a source of light in such a way that the rays of light strike the symmetrical points in the body at equal angles; this is the case when the animal places its median plane in the direction of the rays of light. Points on the dorsal or ventral surface equidistant from the median plane have unequal irritabilities, the irritability being in general the greater the nearer the points are to the oral pole. In the same way, the irritability of a dorsal ele- ment is different from the irritability of the opposite ventral element. If these assumptions regarding the connection between irritability and the main structure of an animal are correct, it follows, without further discussion, that an animal with bilateral symmetry is compelled to place its median plane in the direction of the rays of light and to move in this direction either toward or away from the source of light. We must therefore prove that the described distribu- tion of irritability on the surface of an animal is not fiction, but reality. 1. The oral pole of an animal is more irritable heliotropi- cally than the aboral pole, no matter whether the animal has eyes or not. I have already mentioned that the blind adult Musca larva immediately places the entire median plane of its body in the direction of the rays of light when sunlight strikes only its oral pole. When, however, the oral pole remains in the Digitized by Microsoft® HELIOTROPISM OF ANIMALS 77 shade, while the aboral pole is exposed to the sun, the ani- mal does not bring its median plane into the direction of the rays of light and does not move in this direction, although it may become very restless. Light therefore prescribes the direction of the progressive movements in these animals when it strikes the oral pole, but it does not have this effect when the light falls only upon the aboral pole. T have already called attention to Engelmann’s observa- tion that Euglena viridis reacts clearly only when the light falls upon the oral pole. Kuglena viridis possesses a pig- ment spot at this pole which is called the ‘“eye-spot.” Physiologically this expression is a very unhappy one, as we do not know that this spot has anything in common with the functions of our eye. Engelmann, however, expressly states that the most sensitive spot of the Euglena is not the pig- ment spot, but the colorless protoplasm lying just in front of it. The earthworm has no eyes. Hoffmeister found that the animals recede from the light when the anterior end of the body is illuminated. If, however, the oral pole is shaded and the rest of the body is illuminated, no effect is produced. Darwin repeated and corroborated this experiment.’ Fresh-water Planarians have eyes and are negatively helio- tropic, but their sensitiveness to light is not very great. I cut Planarians in two in the middle. The oral piece reacted to light soon after the operation, just as the whole animal. Not once, however, did I see indications of a heliotropic movement in the aboral piece before regeneration had set in? (which is very complete in these animals), even though the aboral piece was by no means inactive, but crept around very energetically in the glass and reacted promptly when touched. When I placed both pieces on the window 1DARWIN, Die Bildung der Ackererde durch die Thatigkeit der Wiirmer, trans. by CARUS, 1882, pp. 11 ff. 21 observed different facts in an American fresh-water Planarian. [1903] Digitized by Microsoft® 18 STUDIES IN GENERAL PHYSIOLOGY side of the vessel, the oral piece moved toward the room side, but not the aboral piece. When the oral piece moved from the room side toward the window, it soon turned about. Under similar conditions the aboral piece continued to creep until it reached the window. When the vessel containing the animals was carefully reversed, the aboral animal was not affected, but the oral animal immediately moved toward the room side. It can easily be shown that in leeches the head, which contains the eyes, reacts more energetically toward light than the aboral pole. If some small stones are lying on the bottom of a beaker which contains such animals, and the vessel is suddenly illuminated, the animals push their heads under the stones, while the aboral pole remains at rest even though exposed to the light. It is astonishing to notice how long after the illumination the reaction appears. It is not unusual for thirty to seventy seconds to elapse between the illumination and the beginning of the movement. Hoff- meister observed a still longer latent period in the case of the earthworm. It would be unnecessary to show that in animals which possess eyes the oral pole is more sensitive toward light than the aboral. We may therefore accept it as certain that the oral pole of an animal is more sensitive toward light than the aboral, whether the animal does or does not possess eyes. In consequence of this fact, it is difficult for an animal to move perpendicularly or obliquely to rays of light emanating from a sufficiently intense source, for, as the oral pole is more sensitive than the aboral, the former must turn more energetically toward or away from the source of light (depending upon whether the animal is positively or nega- tively heliotropic) than the aboral. 2. The heliotropic irritability is also different on the ventral and dorsal surfaces of a dorsiventral animal. To Digitized by Microsoft® HELIOTROPISM OF ANIMALS 719 my knowledge, no observations on this subject have as yet been made on animals. Planarians and leeches afford an example of the differ- ence between dorsal and ventral irritability. In leeches the ventral surface, which has no eyes, is more sensitive to light than the dorsal surface. It has already been said that this animal leaves the dorsal side of its aboral pole exposed to the light, if only its head is protected from the light. Such animals stick to the side of a beaker, so that their ventral surfaces, which carry the suckers, are directed outward. If diffuse light of a sufficient intensity falls upon the ventral surfaces of the animals, most of them leave the window and move to the room side. The animals then turn their dorsal surfaces to the light. Tn this case, as in all the others, only the more refrangible rays are chiefly active. When the animals are covered with red glass, orientation does not follow, or only after some time. If blue glass is held over them, the orientation takes place just as in diffuse daylight. The difference between the irritability of the ventral and the dorsal surfaces of dorsiventral animals is therefore com- parable with that between the oral and the aboral poles. 3. The dependence of the irritability of a dorsiventral animal on the symmetry of its body must yet be discussed. Those elements of the body of a dorsiventral animal which occupy symmetrical positions with reference to the median plane have equal irritabilities. The facts which prove this are to such an extent objects of daily experience that a brief allusion to them will suffice. Tf a touch on one side of the animal calls forth a movement to the left, then the same stimulus applied to the opposite symmetrical spot on the body will cause the same amount of movement to the right. An object appearing in the right field of vision causes the same amount of movement as one Digitized by Microsoft® 80 STUDIES IN GENERAL PHYSIOLOGY appearing in the left at the same distance from the median plane. In short, we can say that the symmetrical plane of an animal from a morphological standpoint is also the symmetrical plane from a physiological standpoint. This distribution of irritability on the surface of an animal determines the orientation of dorsiventral animals toward a source of light. If the median plane lies in the direction of the rays of light, the symmetrical points of the surface of the animal are struck by the rays at an equal angle. The effects of the stimuli on the right and left halves of the body annihilate each other, since they are equal in intensity and opposite in direction. The light can therefore produce no tendency to turn to the right or the left. When, however, the median plane is oriented obliquely toward the source of light, unequal forces act upon symmetrical ele- ments, and a tendency to turn must arise which continues until the median plane coincides with the direction of the rays of light. This dependence of irritability on the form of the body causes Musca larvee to move away from the source of light precisely in the direction of the rays, and plant lice to move just as precisely in the direction of the rays toward the source of light. The heliotropic movements of an animal are therefore dependent on the symmetrical relations of its body, in the same manner as was shown by Sachs to be the case in plants. To show how far these conceptions of heliotropic phenom- ena in animals differ from the prevailing notions on the subject, especially those of the Darwinians, I shall give the views of Romanes on this subject. Romanes mentions the well-known facts that insects of all kinds fly into the flame, that many birds are attracted by the light of lighthouses, and fishes by the lanterns. He explains the phenomenon as follows: ‘The habit must be attributed to mere curiosity, or Digitized by Microsoft® HELIOTROPISM OF ANIMALS §1 desire to eramine the new and striking object;” and then quotes some remarks which he found in the manuscripts of Darwin: “Query: Why do moths and certain gnats fly into candles, but not into the moon when the same is at the hori- zon? I noticed long ago that they fly much less frequently into candles on a moonlight night. When a cloud passes over the moon, they are again attracted by the candle.” Romanes believes that: “The answer must be that the moon is a familiar object, which insects consider as a matter of course, and so have no desire to examine it.” As we have seen, it is not the “new and striking’’ object and “the desire to examine it” which drive the insects to the lamp, for they are attracted, as I have shown, also by the natural source of light, the sun. No reason seems to exist to my mind for believing that the moon is a more familiar object to the insects than the sun. I cannot well see, however, how Romanes harmonizes the phenomena of negative heliotropism in animals with “the desire to examine unfamiliar objects.” The history of science has taught us that confusion always reigns when anthropomorphic motives are brought into scientific research. Before the time of Galileo a body sinking in a fluid ‘‘sought its place.”’ Galileo and his followers put an end to the sovereignty of this psychology, at least in inanimate nature. Mankind has had no reason to regret this revolution. In biology, however, even at this date, protoplasmic substances still move toward the source of light “because of curiosity.” ‘XIII. SUMMARY OF RESULTS I shall conclude by summarizing the more important results of my investigation: I. The dependence of animal movements on light is in every point the same as the dependence of plant movements on the same source of stimulation. 1See Maca, Geschichte der Mechanik, 1st ed., p. 117. Digitized by Microsoft® 82 STUDIES IN GENERAL PHYSIOLOGY 1. The direction of the median plane, or the direction of the progressive movements of an animal, coincides with the direction of the rays of light.’ 2. The more refrangible rays of the visible spectrum are exclusively or more effective, than the less refrangible rays, in causing the orientation of the animals, as is also the case in plants. 3. Light of a constant intensity acts as a constant stimu- lus in animals as well as in plants. 4, The intensity of the light is of importance in animal heliotropism, in so far as only light of a certain intensity can cause heliotropic movements, and in so far as with an increase in the intensity the orientation of the animals toward the source of light becomes more exact. Direct sunlight causes winged insects (ants, Lepidoptera, plant lice, etc.) to fly, while diffuse light usually causes them only to creep. Posi- tively heliotropic animals will move toward the source of light, even if in so doing they go from places of greater intensity of light to places of less intensity ; negatively helio- tropic animals move away from the source of light, even if in so doing they pass from regions of less intense light to regions of greater intensity. 5. Heliotropic movements occur only between certain limits of temperature. An optimum temperature lies beween these two limits at which the heliotropic movements occur most rapidly and precisely. This holds true also in plants. II. The orientation of an animal toward a source of light depends on the form of the body, just as the orientation of a plant to light depends on the form of the plant. 1, Symmetrical points on the surface of the body of dor- siventral animals possess equal irritabilities.? 1If there is only a single source of light. If there are two sources of light of different intensities, the animal is oriented by the stronger of the two lights. If their intensities be equal, the animal is oriented in such a way as to have symmetrical points of its body struck by the rays at the same angle. [1903] 2 Equal in magnitude, not in direction. [1903] Digitized by Microsoft® HELIOTROPISM OF ANIMALS 83 2. The heliotropic irritability of the oral pole of an ani- mal is different from the irritability of the aboral pole, and is generally greater than the heliotropic irritability of the aboral pole. 3. The irritability of the ventral surface is different from the irritability of the dorsal surface. These three conditions taken together cause dorsiventral animals to place their median planes in the direction of the rays, and to move toward or away from the source of light in this direction. 4, Eyeless animals (such as the larvee of Musca vomitoria) behave in this respect like animals having eyes. III. The heliotropic irritability of an animal manifests itself frequently only at certain epochs of its existence. 1. In winged ants this epoch is the time of the nuptial flight. 2. In plant lice it is the time when wings are present. 3. In the larve of Musca vomitoria negative heliotropism is most prominent when they are fully grown. 4. Ina large number of animals the sense of heliotropism is of the opposite kind in the larval and the adult states. 5. Both the night and day Lepideptera are positively heliotropic, and their heliotropism is similar to that of every other positively heliotropic animal. The period of sleep of the night Lepidoptera, however, falls in the daytime, and only for this reason is their heliotropism manifested exclu- sively at night. IV. In many animals heliotropic irritability is connected with sexuality. Aside from the nuptial flight of ants, the fact must be mentioned here that in ants and Lepidoptera the males are heliotropically more sensitive than the females. V. The behavior of an animal depends on the sum total of its different forms of irritability. In this way it may happen that Cuma Rathkii and the caterpillars of the willow. Digitized by Microsoft® 84 STUDIES IN GENERAL PHYSIOLOGY borer, which live in the dark, are positively heliotropic with- out deriving any benefit from this form of irritability. VI. There is one form of irritability widely distributed throughout the animal kingdom, which has been studied but little, and which can easily be confounded with negative heliotropism. It consists in many animals being compelled to orient their bodies against the surfaces of other solid bodies in a certain way, or bringing their bodies in contact with other solid bodies on as many sides as possible (stere- otropism). Certain animals seek only the concave corners and edges of boxes (Forficula auricularia, ants, Amphipyra, the larvee of Musca vomitoria, etc.); while others fasten themselves only to the convex edges and corners (caterpillars of Porthesia chrysorrhea). VII. A non-luminous source of heat may influence the orientation, but generally it is not able to prescribe the direc- tion of the progressive movements of animals. In this way it happens that animals which move away from a source of heat may be forced by the light to move from diffuse light into sunlight, and to remain exposed to the high temperature of the sunlight, even though this may cause their death. The influence of a non-luminous source of heat can best be compared to the influence of a weak source of light, which is just sufficient to hinder a negatively heliotropic animal from going toward the source of light, but is not sufficient to force the animal to move accurately in the direction of the rays. We have yet to draw a conclusion from the results of these experiments, which could not be formulated until now. We have seen that the heliotropic movements of animals possess- ing a nervous system are determined in all respects by the same external conditions and depend in the same way on the external form of the body as do the heliotropic movements of plants, which have no nervous system. These heliotropic phenomena cannot therefore depend upon specific character- estics of the central nervous system. Digitized by Microsoft® HELIOTROPISM OF ANIMALS 85 XIV. ADDENDUM: SOME FURTHER EXPERIMENTS ON THE GEOTROPISM OF INSECTS As I have several times had occasion in this volume to mention the influence of gravity on the orientation of ani- mals—a subject which in this form has not yet been discussed in physiological literature—it may perhaps be desirable to add a few further facts on animal geotropism. I must say beforehand, however, that my experiments in this field are not yet completed, and that I intend to return to this sub- ject. 1. I have found that caterpillars (for example, Bombyx neustria) when placed in a hollow vessel creep vertically upward. When we wish to pour such caterpillars out of a vessel, we employ a method opposite to that used in pouring out a liquid ; we must hold the mouth of the vessel upward. When such caterpillars are contained in a glass vessel, the diffuse daylight entering from above in itself would bring about this effect; I therefore made this experiment in wooden vessels. When the opening of the vessel was directed downward the animals crept upward, and not an animal escaped from the vessel. Geotropism, however, like heliotropism, is especially evident only at certain epochs in the life of the animal; for the geotropic experiments were not at all times successful even in the same animals. 2. Small beetles, particularly Coccinella, which can always be procured with ease in great numbers, were placed in a wooden box, and to protect them from the effects of light I put the box in a dark closet. The animals, which were at first scattered over the whole box, were found the next day collected at the highest point in the box, on the upper side, where they remained. When I turned the box about, they changed their orientation and moved again to the top. The behavior of Coccinellide and other beetles (particularly leaf Digitized by Microsoft® 86 STUDIES IN GENERAL PHYSIOLOGY beetles) is somewhat remarkable, as many other animals, such as caterpillars, plant lice, etc., do not react geotropically after they have been kept in the dark for some time. 3. Another phenomenon can be observed in Coccinellide, to which I have already referred in my former paper on the “Orientation of Animals toward the Center of Gravity of the Earth.’’! Cockroaches, for example, usually assume a position on the vertical walls of boxes ; they never remain for any length of time on the horizontal bottom, where their ventral surfaces are turned toward the center of gravity. If, for example, the roaches are scattered about on the four vertical walls of a box, and the box is carefully and slowly turned through an angle of 90° on a horizontal axis, only those animals whose ventral surfaces are turned toward the center of gravity in the new position begin to move. They leave the plane on which they were up to the time the box was turned and again seek a vertical plane from which to suspend themselves. Yet the animals on the other three vertical walls remain at rest while the box is turned. This seems to indicate that the animals cannot turn their ventral surfaces toward the center of gravity for a long time. Whether this means that the animals’ extremities can bear the pull of the load of the body better than its pressure, I cannot say. The same phenome- non is also observed in Coccinellide. If the box is placed obliquely, the two oblique planes on which the animals turn their ventral surfaces toward the center of gravity are first abandoned. 4. Aside from the peculiarity which has been mentioned, the Coccinellide, like roaches, are found hanging on walls in all possible orientations toward the center of gravity of the earth. Great differences are therefore found in this regard also, for there are animals (such as the newly hatched Lepidoptera) which put the median plane of their bodies in 1Sitzungsberichte der Wtrzburger physikalisch-medicinischen Gesellschaft, 1888. Digitized by Microsoft® HELIOTROPISM OF ANIMALS 8&7 the direction of the vertical and turn their heads upward. There are, however, also animals which orient themselves in exactly the reverse manner and turn their heads downward. To this class belongs the garden spider, which we find hang- ing in this position in the center of its web for hours. I found the same behavior in one of the Diptera, which I have not yet classified. 5. If a beetle or a house fly (from which the wings have been removed) is placed on the disk of a centrifugal machine, which is rotated slowly, the insect turns its body around the same axis, but in an opposite direction to that of the revolv- ing plate. If the velocity of the machine is increased, these compen- satory movements cease. These animals therefore behave in this respect exactly as Mach claims vertebrates behave which possess a labyrinth.’ But while the movements of vertebrates continue for some time after the movement of the centrifugal machine has ceased, but in a sense opposite to those occur- ring during rotation, I have never been able to bring about these compensatory after-movements in insects.” 6. When one hemisphere of the brain of a house fly is removed the same disturbances in orientation appear as after the same operation on a rabbit. The fly from which the left hemisphere has been removed moves continually toward the right in its progressive movements. These deviations are greater or less according to the success of the operation. I showed in an earlier paper that the turn-table reactions of dogs and rabbits deprived of a hemisphere might be unsym- metrical. If a fly which has lost the left half of its brain is placed on a rotating disk, we find that on turning the disk in the direction of the hands of a watch as seen from above, 1Macu, Grundlinien der Lehre von den Bewegungsempfindungen (Leipzig, 1875). 2The observations of Lyon and of Radl suggest the possibility that these phe- nomena depend upon the eyes of these animals, When the eyes are removed or black- ened the reactions cease. [1903] Digitized by Microsoft® 88 STUDIES IN GENERAL PHYSIOLOGY the fly makes slight or only weak compensatory movements Yet when the disk is turned in the opposite direction the fly reacts very promptly (apparently even better than before the operation). After destroying both hemispheres or ampu- tating the head I no longer obtained compensatory move- ments in the fly. The experiments of Mach show that the compensatory movements of vertebrates emanate from the head ; according to Mach, the labyrinth is to be considered the essential organ. Such an organ, however, does not exist in the head of a fly. 7. If the halteres are removed from a fly, it can no longer fly upward ; in the attempt to fly it immediately falls to the ground, where it frequently tumbles about. Gleichen- Russwurm established this fact during the last century. I found that such a fly reacts normally on the centrifugal machine. The destruction of the halteres does not there- fore have the same effect as the destruction of the labyrinth in frogs, birds, or mammals, in which, according to the experiments of Hégies and Schrader, compensatory move- ments cease when the labyrinth is destroyed. The conjec- ture expressed by others, and by me in my first publication, that the direction of sound has an influence on orientation has thus far led me to no new facts. 8. I have not yet been able to demonstrate compensatory movements on the centrifugal machine in caterpillars, Musca larvee, and snails. Digitized by Microsoft® II FURTHER INVESTIGATIONS ON THE HELIOTROPISM OF ANIMALS AND ITS IDENTITY WITH THE HELI- OTROPISM OF PLANTS! In a former paper I showed that the dependence of animal movement upon light is identical with that of plants on the same source of stimulation.” I showed that the law put forward by Sachs for the heliotropism of plants, namely, that the direction of the rays of light deter- mines the orientation, holds good also for animals. Free- moving animals are compelled to execute their progressive movements in the direction of the rays of light, as is the case with the swarm-spores of certain Algez. It was further proved that the more refrangible rays of the visible spectrum are the rays that are solely, or at least chiefly, effective in bringing about the movements of these animals; as is the case in the heliotropic movement of plants. After I had proved the identity of this relationship point for point, I believed it permissible to designate these reactions of ani- mals by the same term as that used for the identical reactions of plants, namely, ‘“heliotropism.” At that time, however, I had proved this identity only in the case of free-moving animals. The task still remained to ascertain and investigate the influence of light upon the orientation of sessile animals, and to decide whether the influence of the light is in this case also similar to that upon sessile plants. It is known that in plants the direction of the 1 Pfliigers Archiv, Vol. XLVII (1890), p. 391. 2PartI,p.1. See also Logs, Sitzungsberichte der Wirzburger physikalisch- medicinischen Gesellschaft (1888) ; GRooM UND LOEB, Biologisches Centralblatt, Vol. X (1890). 89 Digitized by Microsoft® 90 Srupres IN GENERAL PHYSIOLOGY light rays determines the orientation of the organs of the plant. It is characteristic of the organs of sessile plants that heliotropic curvatures are produced when the plant is illu- minated from but one side. A growing stem continues to bend when illuminated from one side only until the growing tip lies in the direction of the rays of light. Progressive movement in the direction of the rays of light, which is the rule for free-moving animals and plants, is of course impos- sible for sessile organisms. Everyone who has cultivated flowers in a room has no doubt observed the heliotropic bendings in the plants. The question now arises whether these heliotropic curvatures can also be produced in sessile animals when illuminated from one side only. I shall show in the following pages that this is, indeed, the case. I 1. The experiments described here were made on the large marine Annelid, Spirographis Spallanzanii. It lives in a tube which is quite flexible, yet sufficiently rigid to keep the ani- mal in a definite position. The tube is formed from the secretions of the animal. The aboral end of the tube is fastened (by a secretion) to stones or other solid objects. The gills of the animal, which are arranged at its anterior end in several spiral turns radial to the longitudinal axis of the animal, are usually found unfolded and projecting beyond the open end of the tube. As the tube is almost impervious to light, the latter will act chiefly upon the gills. So far as we know at present, the animal has no eyes. The animal can move freely inside the tube, the inner surface of which is perfectly smooth, and can be removed from it without the slightest injury by cutting open the tube. I have occasionally seen the worm leave the tube of its own accord, when the water in the aquarium became bad. The layman seeing these animals in the tubes with their Digitized by Microsoft® FURTHER INVESTIGATIONS ON HELIOTROPISM 91 gills fully unfolded takes them at first for plants bearing a palm-like crown (the gills) upon a long naked stem (the tube). A slight jar, however, causes the animals to draw back their gills rapidly into the tubes. When the animal is taken from the sea and kept in an aquarium, it is at first indifferent to the light. This con- tinues until the animal has attached itself by its foot to the bottom of the aquarium—a period often of several days. As soon as this has taken place, however, the orienting influence of the light begins to be noticeable. If light falls upon the animal from one side only, heliotropic curvatures make their appearance in the tube. The animal turns its oral pole toward the source of light and bends its tube until the axis of its radially expanded gills lies in the direction of the rays of light. The animal maintains this orientation as long as the direction of the rays of light remains unaltered. 2. To test more accurately to what extent the direction of the rays of light determines the orientation of the animals, I put them into an aquarium which stood at the window, and which could be completely screened from the light by a zine box. The outlines of the aquarium are indicated in the drawings (Figs. 7 and 8) by black lines, the outlines of the zine box by dotted lines. The wall abcd of the zinc box could be moved vertically upward, so that the amount of light entering the aquarium could be regulated. The zinc box, the walls of which were painted black on the inside, was so placed over the aquarium that the movable wall was on the window side of the aquarium. If this wall was raised only slightly, as shown in Fig. 8, the rays entered the aquarium almost horizontally. When it was drawn farther up, as in Fig. 7, rays entered from above in addition to the horizontal rays. These were more intense than the rays entering horizontally. On December 14, 1889, I put nine vigorous specimens of Digitized by Microsoft® 92 STUDIES IN GENERAL PHYSIOLOGY Spirographis Spallanzanii, each about 15 cm. long, on the bottom of the aquarium, with the longitudinal axes of their tubes perpendicular to the plane of the window. Hight of them lay with their oral poles toward the room side efgh (Fig. 7) of the aquarium; one with its oral pole toward the window side. The first two days passed without any change Ss noon Sy e é FIG. 7 in the orientation; the animals first attached the aboral ends of their tubes to the floor of the aquarium. In the course of the third day the tubes of six of the animals, which were placed with their oral pole toward the room side, began to bend in an almost horizontal plane, the concavity of the curv- ature being directed toward the window. The other two animals, which had likewise been placed with their heads toward the room side, first elevated the head end and then curved the tube concavely toward the window. Finally, the ninth animal, which I had placed in the aquarium with its head toward the window, raised its head a little. Digitized by Microsoft® FURTHER INVESTIGATIONS ON HELIOTROPISM 93 Within the next few days the six first-mentioned animals further elevated their heads, so that the animals on Decem- ber 22—eight days after being placed in the aquarium— were all similarly oriented toward the light. The head was directed toward the window, and the axis of symmetry of the gills which were exposed to the light lay in the direction of the more intense rays of daylight which entered from FIG. 8 without and above. I waited to discover whether this orien- tation would last. The aquarium remained undisturbed until February 16, 1890; that is, for more than two months, The animals also did not change their positions, as indicated in Fig. 7. 3. On the afternoon of February 17, 1890, the aquarium was turned 180° about its vertical axis, and the zinc box was again inverted over the aquarium so that the movable end was directed toward the window. By turning the aquarium around in this way, the heads of the animals, which had been until then directed toward the source of light, were suddenly turned toward the room side of the Digitized by Microsoft® 94. STUDIES IN GENERAL PHYSIOLOGY aquarium. My object in turning the aquarium around was to see whether a change in the direction of the rays of light would cause the animals to reverse their heliotropic curva- tures and to turn their heads again toward the source of light. There was no change during the course of the afternoon and night. But toward noon of the following day I found two animals, which in the morning had still been in the position AB (Fig. 9), in the position AB, ; F indicates the plane of the window. The portion DB of the tube had described the surface DBB, about the point Dascenter. A similar change in the orientation of all the remaining animals took place during that and the following day. In this experiment the direction of the rays of light was modified somewhat; the wall abcd was left quite low, so that almost nothing but horizontal rays entered the aquarium (Fig. 8). Iwished to determine whether the animals would continue to follow the direction of the rays and so assume an almost horizontal position. This did, indeed, occur. On February 22, 1890, five days after reversing the aquarium, the orientation was accomplished, as indicated in Fig. 8. The animals had turned their heads toward the source of light, and the axes of their gills lay almost horizontally in the direction of the rays of light. I left the conditions of the experiment unchanged until toward the end of March, and during all that time the animals maintained their orientation. 4. If the rays of light fall vertically from above into the aquarium, Spirographis directs its tube vertically upward, exactly as a stem grows vertically upward into the open air. FIG. 9 Digitized by Microsoft® FurtuHeR INVESTIGATIONS ON HELIOTROPISM 95 This experiment was made in another aquarium, in which the light rays entered chiefly from above. The animals in the large aquarium of the zodlogical station at Naples are usually found mainly in this position; the light enters this aquarium chiefly from above. Here, however, where free- swimming forms easily disturb the orientation of Spiro- graphis, it is not always so perfect as when all possible dis- turbing causes are avoided, as in an aquarium used only for such experiments. 5. It follows from these experiments that gravitation exerts only a slight effect, if any, upon animals which are subjected simultaneously to the effects of light and gravity. It was, however, necessary to discover whether a geotropic erection of the animals would not occur under the influence of gravity alone in a completely darkened room. On March 21, 1890, I placed a large number of Spiro- graphis in a horizontal position upon the floor of an aquarium in the dark room. On March 24 most of the animals had attached themselves by their aboral ends to the bottom of the aquarium. The oral ends of the tubes were then elevated until the gills no longer touched the bottom of the aquarium. The axis of the spiral did not stand vertically (as was the case when light fell vertically into the aquarium, or as should have been the case had the animals been geotropically irri- table), but only at a slight angle from the horizontal. The animals remained in this position until the end of the experi- ment, which was interrupted in the middle of April. Gravity therefore has no important influence upon the orientation of Spirographis Spallanzanii. 6. The contact-irritability of the gills is manifested by the fact that they bend away from solid surfaces. This form of irritability can modify the result of the heliotropic experiments upon the animals. I placed several of the ani- mals upon the floor of an aquarium which was so shallow Digitized by Microsoft® 96 STUDIES IN GENERAL PHYSIOLOGY that the animals could not erect themselves. They were so placed in the aquarium that their longitudinal axes lay per- pendicular to the side ab (Fig. 10) of the aquarium, and their pedal extremities JZ touched the glass wall ab. The side a faced and was parallel to the plane of the window. The animals fastened a ! M 3 themselves to the wall ab, and then began to react, in their char- acteristic way, to the light, by which the head was turned and the tube became con- mi cave toward the FIG. 10 source of light. The tube ALN assumed the position MN,. As soon, however, as the tentacles touched the glass wall ab, the tip NV turned away from the glass wall. The heliotropic bending gradually affected all the elements of the tube MN, so that the Spirographis finally reached the position MN,, in which it remained throughout the period of observation—four months, I repeated this experiment a number of times, always with the same result. 7. The heliotropic phenomena of Spirographis took place both in direct sunlight and in diffuse daylight. The minimum light intensity just sufficient to bring about these phenomena is very small. I have not yet studied the effect of rays of different refrangibility in producing these phenomena. Since thus far the more refrangible rays have proved to be the most effective heliotropically both in plants and animals, it is to be suspected that Spirographis also will prove no excep- tion. 8. As is well known, Sachs has formulated the law that SS & Digitized by Microsoft® FuRTHER INVESTIGATIONS ON HELIOTROPISM 97 radial plants are orthotropic; 7. ¢., they place their longi- tudinal axes in the direction of the rays of light, or of gravity. It will have occurred to the reader that Spiro- graphis, the body of which, like that of all Annelids, is built on the dorsiventral and not on the radial plan, reacts toward the light as a radial plant organ. I have, however, already emphasized the fact that only the radially arranged gills of the animal are exposed to the light, while the remainder of the animal is inclosed in the tube. These observations, therefore, show that a radial animal organ also obeys the law of orientation established by Sachs for plants (even though Spirographis possesses a central nervous system, which the plant does not). It is also of physiological interest that the respiratory organ of Spirographis is so highly sensitive to light that the orientation of the whole animal in space depends essentially upon this sensitiveness. This fact may perhaps explain why Branchiomma, a Serpulida quite similar in structure to Spirographis, has well-developed eyes upon its gills. 9. If Spirographis is carefully removed from its tube, it is not able to raise its body from the floor. In such a con- dition it creeps about like an earthworm, only much more slowly. I have occasionally seen such animals creep to the window side of the aquarium. They appeared, however, to suffer from contact stimuli, to which they were constantly exposed in this condition ; they all died within a few days. 10. I am not in a position to make a definite statement concerning the mechanics by which the heliotropic curvature of the tube is brought about in these animals. The wall of the tube of an adult animal is 1.25-1.5 mm. or more thick. It is very flexible and elastic. If the animal ts taken out of the tube after the latter has been bent through the heliotropic reactions of the animal, the tube nevertheless maintains tts Digitized by Microsoft® 98 STUDIES IN GENERAL PHYSIOLOGY curvature. The wall on the outer concave side of the tube is therefore permanently shortened. It might seem that the limit of elasticity of the tube is so low that it retains, like a piece of lead, a curvature imparted to it through the muscular force of the animal. But thisis not the case. I puta thick rod of lead into the straight tube of a Spirographis and bent it till the tube was strongly curved. The lead rod was allowed to remain in the tube. When a week afterward I withdrew the rod from the tube, it retained only a trace of the curve impressed upon it. Similar failure followed my attempt to straighten by the same method, a heliotropically curved tube. Yet, as I have already shown, Spirographis is able to straighten its curved tube within a few hours after a change in the direction of the rays of light, and, what is more, the tube remains straight. The tube retains its curvature even after it has been split open. The animal has, however, besides pressure and pull, another means at its disposal to change permanently the orientation of the tube, namely, the production of a secretion and the formation of a new layer within the tube. The idea that permanence in the curvature is attained in this way is supported by the fact that the inner layer of the tube is much more elastic than the outer layers, so that the formation of a new inner layer on one side of the tube might curve it permanently. The following fact supports this view: If a tube is cut open lengthwise, the cut margins roll inward. If the individual layers are separated, as can be done easily, it is seen that the tendency of the inner layers to curl up is greater than that of the outer layers, and that of the innermost, newest layer is the greatest of all. The formation of a new inner layer on one side of the tube would, therefore, be sufficient to maintain the curvature of the tube permanently. The formation of a new layer cannot be observed directly. One is also disap- pointed in the hope of finding one side of the wall of the Digitized by Microsoft® FuRTHER INVESTIGATIONS ON HELIOTROPISM 99 tube thicker than the other, for the thickness of the wall of perfectly straight tubes varies greatly in different places of the same cross-section. The thickness of the wall is therefore no criterion in answering our question. I can therefore formu- late the following theory of the origin of the heliotropic curvature in the tube, only by reserving the right to test, and perhaps modify it later. I believe that, when illuminated from one side only, the animal strives at first to bring the axis of its gills as nearly as possible in the direction of the rays of light. In do- ing so the animal perhaps bends the tube by aid of its muscular force. Since the tube, however, tends to resume its original position because of its elasticity, the body of the Spirographis must rub more strongly against the concave wall of the tube than against the other. This increased friction brings about a great activity of the skin glands, whose secretion forms the material of the tube. That friction indeed leads to secretion, and with it to the formation of a tube, I have been able to prove directly in the case of the Actinian, Cerianthus membranaceus. I have been able to establish the following facts regarding Spirographis which seem to indicate a similar behavior. I cut small pieces from the tube. The animal was in conse- quence obliged to rub against the cut margins during its movements ; and a copious secretion was indeed formed in a short time, which soon closed the opening with a new mem- brane. There is, moreover, always more or less friction on the anterior margin of the tube when the animal stretches out its head. In fact, the tube grows constantly from this Digitized by Microsoft® 100 SruDIES IN GENERAL PHYSIOLOGY end, as illustrated in Fig. 11. In this experiment I had cut a long broad piece aa, out of the tube at a, so that the anterior piece of the tube a,b remained attached to the rest of the tube only by a thin piece p. After the operation the animal showed its gills at a, and no longer used the piece a,b of the tube. New material was deposited at a within a few days, and in the course of three weeks the new piece ac was formed. Its light color readily characterized it as new. I had at the same time cut away the aboral end of the tube completely. Before my very eyes the movement of the aboral end upon the sand caused the secretion of a sticky mass, to which particles of sand became attached. In this manner the new piece of tube de was built, consisting of grains of sand cemented together by the glandular secretion from the tube. The newly formed piece was perfectly smooth on the inside. The secretion from the skin glands continues as long as there is any noticeable amount of fric- tion. When I removed Spirographis from its tube and placed it in a smooth test-tube, practically no secretion occurred. Secretion occurred only from the parapodia in the form of long, fine threads, similar to those produced by the spinning glands of spiders. If, however, the naked Spirographis was laid upon the sand, the aboral end was soon covered by a shell of sand kernels. I have never, however, seen the animals form a complete tube when removed from their old ones ; for in their exposed condition they soon die. II Spirographis Spallanzanii attains its heliotropic orienta- tion when illuminated from one side by curving its flexible tube; new growth of the tube is not necessary. There are other Serpulide, however, the calcareous tubes of which are stiff and inflexible. These Serpulide, like Spirographis, expose their gills to the light, and these, too, react according Digitized by Microsoft® FuRTHER INVESTIGATIONS ON HEtiotTRopPIsm 101 to their structure as radial organs. Such a Serpulida, if heliotropically irritable, must place the longitudinal axis of its cylindrical tube in the direction of the rays of light. If the calcareous tube is brought into any other position with reference to the source of light, the animal must make use of one of two possibilities in order to regain its proper orien- tation: either it must lengthen its calcareous tube and bend the newly growing part until the axis of the tube again a<—G pases Ca i dg eas BOL HIT CCIE Crt TH. rr MULL MLE COUr ee CCEA ee ae rg TET e (Ror Create tssatraatin EG SST PCT ane Tah ae ee oH Ci FIG. 12 lies in the direction of the rays of light, or else leave its tube entirely and build a new one having the proper orientation. The animal makes use of the first of these possibilities. I experimented with Serpula uncinata. These Annelids inhabit calcareous tubes and are gregarious. Large white blocks are found in the Gulf of Naples which consist entirely of the tubes of countless numbers of such Annelids massed together. I noticed that the individual tubes in such a mass all had the same orientation, and in those cases in which the blocks showed the base upon which they had rested on the horizontal bottom of the ocean it was plainly visible that the tubes must have stood in the water with their longitudinal axes vertical. Serpula can, like Spirographis, move about freely within its tube. I laid a large block of innumerable annelid tubes, each of which Digitized by Microsoft® 102 STUDIES IN GENERAL PHYSIOLOGY stood almost mathematically straight and parallel, upon the floor of the aquarium so that the longitudinal axes ab of the tubes, which had previously been vertical, now had a hori- zontal position (Fig. 12). The light fell into the aquarium from above. I noticed that in the course of the next day the Serpulide, which like Spirographis presented only their radially arranged gills to the light, bent them strongly upward. Individual tubes then began to grow, and in such away that the newly formed portions of the tubes all bent upward until the free tip of the tube lay in the direction of the rays of light (which in this case was identical with the direction of gravity), afler which the tubes continued to grow in the direction of the rays of light (and of gravity). Within six weeks the entire block was covered with tubes which curved upward; not a single individual had continued to grow in the original direction ab. The figure shows the Serpulide curving upward at the free edge of the block. The final effect in this case therefore again corresponds to the theory of geotropism and heliotropism as presented by Sachs: the axis of the gills which react as a radial organ lies finally in the direction of the rays of light (and of gravity). While in the case of Spirographis, however (the tube of which is flexible), this effect was brought about through a change in the orientation of the old tube, the same effect was attained in the case of Serpula (the tube of which is inflexible) only through the heliotropic curvature of that portion of the tube which was in the process of growth. In the above-mentioned experiment the direction of the light rays was identical with the direction of gravity. I have not yet been able to decide whether light alone deter- mines the orientation of the tube, or whether gravity also plays a rdle. I hope later to make a series of experiments regarding this point. Digitized by Microsoft® FuRTHER INVESTIGATIONS ON HELIOTROPISM 108 III 1. I have endeavored to find other animals in which helio- tropic curvatures are formed only in the growing parts. These efforts have been successful in the Hydroids. Stems of Sertularia (polyzonias ?) were cut off near the root and fixed in the sand in an inverted position, so that the cut end was directed upward. The stems were placed near a window through which the light fell obliquely and from above. The animals began to regenerate; new polyp-bearing stems grew from the cut end as well as new roots ;* but while the new stems grew upward and toward the window, the roots grew downward and toward the room side. The polyp-bearing shoots are positively, the roots negatively, heliotropic. That the negatively heliotropic elements were true roots was proved by the fact that when brought in contact with a solid body they attached themselves to it and continued to grow over its surface in close contact with it. They could be loosened from their attachments only by force. The polyp- bearing stems do not possess this kind of contact-irritability. The heliotropic phenomena will be readily understood by the aid of Figs. 13, 14, and 15: ab is the old stem, 6 the cut end; the stem is fixed in the sand to the point ac. From the cut end b arise newly formed roots W,, which bend down- ward away from the light and toward the room side of the aquarium. The new polyp-bearing shoots S grow upward and toward the window. The arrow marks the direction of the rays of diffuse daylight in this experiment. 2. In these experiments new growths occasionally sprang from the middle of the old stem, which, so far as their con- tact irritability was concerned, reacted as roots. Those tendrils which attached themselves to solid bodies were always negatively heliotropic. They grew downward and 1 Which is of importance in the theory of organization. Digitized by Microsoft® 104 STUDIES IN GENERAL PHYSIOLOGY toward the room side, and remained free of Hydranths. (See Fig. 13, W,.) On the other hand, I saw also new polyp-bearing stems arise from the old stems, although much less frequently; these grew in the opposite direction, namely upward. 3. That in the case of Sertularia it is, indeed, only the growing parts which produce the heliotropic curvatures is FIG. 13 FIG. 14 FIG. 15 shown by the following experiment. The growing tips were cut off a large number of Sertularia stems. The stems began to grow, and in the course of a few days sent out new sprouts. The new growth is strikingly different in color from the old stem; while the latter is rather brown (from having been covered by Algz ?), the color of the new growths is a light yellow. The growing elements curved themselves until the growing points lay in the direction of the rays of light and then continued to grow in this direction. During all this time no change in the orientation of the old stem occurred, nor did any take place in other uninjured stems, in which no linear growth occurred during this time. How far gravity played a role in these experiments I was Digitized by Microsoft® FURTHER INVESTIGATIONS ON Hexriorropism 105 unable to determine accurately. The Sertularia cultivated in a dark room ceased to grow, though I question whether this was entirely due to lack of light. 4. Light (and perhaps gravity) influences not only the orientation, but also the position of the newly formed organs. I have observed, and not in the case of Sertularia only, that the new polyp-bearing branches always arise from the upper surface of the stem. In Fig. 15, a new stem S springs from the upper side (the side directed toward the source of light) of the stolon W,. I do not desire to discuss these points more minutely here, as they will form the basis of a paper which is to appear soon, on the form of animals. The experiments on Sertularia described here serve only to complete the general consideration of animal heliotropism and to show more fully the identity of animal and plant heliotropism. The special investigation of the heliotropic behavior of Hydroids is to be the subject of future study. That this is both an interesting and a fruitful field is shown by the beautiful work of Hans Driesch, which has just appeared, on the “ Heliotropism of Hydroids.”’ Driesch arrives at the following result: The stolons which are produced instead of polyps under unfavoraBle conditions in Sertularella polyzonias, are with the exception of the first, which is turned away from the light from the very beginning, all positively heliotropic at first, becoming nega- tively heliotropic after the growth of the daughter-stolons. They arise from the side of the mother-stolon, which is turned toward the light. (P. 152.) This observation of Driesch agrees very well with mine. I shall return to them in my ‘Physiological Morphology of Animals.” The results of this study may be summarized as follows: 1. Certain sessile animals (Serpulide, Hydroids) which are compelled to react to light and gravity as radial organ- 1 Zoologische Jahrbticher, Vol. V. Digitized by Microsoft® 106 STUDIES IN GENERAL PHYSIOLOGY isms always place the axes of their radial organs in the direction of the light rays, as do the radial organs of sessile plants. 2. The fact that sessile animals, such as the Serpulide, have a central nervous system, while plants have not, does not bring about any difference in the heliotropic effect. 3. Ifthe light enters from one side, there are produced in the above-mentioned animals heliotropic curvatures which correspond to those obtained in sessile plant organs under similar conditions. 4. There are sessile animals which attain these helio- tropic curvatures only during the period of growth, as is the case with certain plants. Sertularia and Eudendrium, among others, belong in this group, in which only the growing parts are able to bend heliotropically ; Serpula uncinata, which is able to change the orientation of its otherwise stiff tube only when the latter is growing, also belongs in this group. 5. Spirographis Spallanzanii, the tube of which is flexible, is capable of heliotropic curvatures without accompanying phenomena of growth, as are also certain jointed plant organs which attain their heliotropic orientation without phenomena of growth. Although I do not consider my study of animal heli- otropism ended with this paper, yet I think I have shown that the heliotropism of sessile animals is essentially identi- cal with the heliotropism of sessile plants. Digitized by Microsoft® Il ON INSTINCT AND WILL IN ANIMALS! 1. In the biological literature one still finds authors who treat the “instinct” or the “will” of animals as a circum- stances which determines motions, so that the scientist who enters the region of animated nature encounters an entirely new category of causes, such as are said continually to pro- duce before our eyes great effects, without it being possible for an engineer ever to make use of these causes in the physi- cal world. “Instinct” and “will” in animals, as causes which determine movements, stand upon the same plane as the supernatural powers of theologians, which are also said to determine motions, but upon which an engineer could not well rely. My investigations on the heliotropism of animals led me to analyze in a few cases the conditions which determine the apparently accidental direction of animal movements which, according to traditional notions, are called voluntary or instinctive. Wherever I have thus far investigated the cause of such ‘‘voluntary” or “instinctive” movements in animals, I have without exception discovered such circum- stances at work as are known in inanimate nature as deter- mining movements. By the help of these causes it is pos- sible to control the ‘“‘voluntary” movements of a living ani- mal just as securely and unequivocally as the engineer has been able to control the movements in inanimate nature. What has been taken for the effect of “will” or “instinct” is in reality the effect of light, of gravity, of friction, of chemical forces, etc. The following may be added by way of fuller explanation : 1 Pfliigers Archiv, Vol. XLVITI (1899), p. 407. 107 Digitized by Microsoft® 108 STUDIES IN GENERAL PHYSIOLOGY The position which the tube of Spirographis Spallanzanii assumes in space is such, as we have seen, that the animal turns its oral pole toward the light, and puts the axis of its radial gills into the direction of the rays of light. The direc- tion of the rays of light is the condition which determines the orientation of these animals unequivocally. If the ques- tion should arise as to how to hold a great number of living Spirographes continually and voluntarily in a definite position in space, this could be done, as our investigations have shown, by simply allowing the rays of light to fall upon the animals in the direction which we wish the animals to assume end hold. If anyone endeavors to compel Spiro- graphis to assume a definite spatial orientation either through “instinct” or “will,” he will be obliged to seek the aid of the rays of light in order to obtain the desired result, even if he afterward believes that, beside, before, behind, after, or between the light rays the “instinct” or “will” of the ani- mal co-operated with the light to bring about the move- ment. He will further be able to convince himself that the direction of the light, if sufficiently intense, is alone and unequivocally able to determine the orientation. The direction of the “voluntary” movements of the winged plant lice is determined by the direction of the rays of light. The animals are forced to turn their oral poles toward the light and to move in the direction of the rays of light. If the animals are introduced into a transparent vessel, they live and die on the side of the vessel which is turned toward the light. If anyone should wish to force these animals to move in a fixed direction toward a definite point “voluntarily,” he knows now how this may be accom- plished. He need only allow sufficiently intense light to fall upon the animals in the direction in which it is wished that they should go. As is well known, the direction of the rays of light, par- Digitized by Microsoft® On INSTINCT AND WILL IN ANIMALS 109 ticularly that of the more refrangible ones, determines also the orientation of the organs of a plant. By the help of light the botanist controls the orientation of a plant at will. Why should he maintain that the “will” or the “instinct” of the plant co-operates with the rays of light when the orientation is determined solely and unequivocally by the latter? The movements of an animal toward the light are, however, as I have shown, identical point for point with the movement of a plant toward the light. Wherever the orientation of plants has been satisfactorily controlled experi- mentally, light has, indeed, been considered the sole deter- mining factor; but in the case of animals, in which in similar experiments light is without doubt also the sole determining factor, ‘‘instinct” and. “free will’’ have still been considered to play a réle. Just as the direction of the rays of light (particularly that of the more refrangible ones) is the essential factor in the ex- amples described above, and in many others given in my papers on heliotropism, so in other cases it is gravity, in others again contact with solid bodies, in still others chemical forces, etce., which determine the movements of the animals. 2. In order to state the cause which determines in each instance the “voluntary” movements of an animal, I desig- nated the movements by their external cause. I spoke therefore, as has long been the custom in plant physiology, of heliotropism when the direction of the rays of light determines the direction of the movements of an animal or its orientation ; of geotropism, when gravity, or of stereot- ropism, when contact with solid bodies, determines the orientation or the movements ; etc. A zodlogist asked me reproachfully what had been gained by designating as ‘“‘stereotropism” what had been designated as “instinct.” I was discussing the fact that certain ani- mals creep into the crevices of solid bodies, and the zodlogist Digitized by Microsoft® 110 STUDIES IN GENERAL PHYSIOLOGY was of the opinion that the animal behaves thus through “instinct.” If a physicist finds that liquids rise in a capil- lary, or that one liquid forms a convex while another a concave, meniscus ina glass tube, he will be less easily satisfied than the zoologist, according to whom everything is done through “instinct.” The physicist will endeavor to discover more precisely what conditions underlie the phenomenon. This, it seems to me, is also the problem of the biologist—a prob- lem which ts not even recognized, much less solved, by saying the cause of such or such a motion is an “instinct.” From a biological standpoint one would at first take it for granted that light causes animals to creep into crevices. But I was able to show that the animals creep into the crevices between solid bodies even when the solid bodies are perfectly transparent and are exposed to a strong light; secondly, that the animals behave in a similar way when put in a perfectly dark room. Light is not, therefore, the physical cause which determines this phenomenon. I proved this for Forficula, ants, the larve of Musca vomitoria, etc. Plateau had previously established this fact by a similar experiment upon Cryptops, with which I was not familiar at that time, however. The animals creep into narrow crevices, therefore, not because of the light, but because they are forced to bring as much of their bodies as possible in contact with solid bodies. The friction and the pressure produced by the solid bodies are therefore the determining cause. This view, that light has nothing to do with the phenomenon, but that it is the friction produced by contact with solid bodies, has this advantage over the traditional phrase “It is instinct,” that pressure and friction are physical agencies which, like light, can be controlled quantitatively and qualitatively, and by which we can prescribe unequivocally the “voluntary” movements and the “voluntary” orientation of an animal. I will here add that, while there are a large number of Digitized by Microsoft® On INSTINCT AND WILL IN ANIMALS 111 animals which are forced to bring their bodies in contact with solid objects on all sides as far as possible, there are others which show exactly the opposite form of irrita- bility and immediately draw themselves away from a solid body with which they chance to have come in contact. To these belong the Nauplii of Balanus perforatus, the tiny Mysidex of the Bay of Naples, the gills of Spirographis Spallan- zanii, etc. That that form of irritability which I have called “stereotropism” plays a prominent rdle in life- phenomena, however, follows from the fact that the entrance of the spermatozoon into the egg (as shown by the investi- gations of Dewitz’) is governed by this form of irritability, and that the migration of leucocytes is likewise determined largely by contact-irritability. I have, moreover, inciden- tally found, in my investigations on the influence of external stimuli upon the form of the body, that stereotropism influ- ences not only the shape, but also the size and velocity, of the growth of certain organs. These investigations were made upon Hydroids. I succeeded in producing stercotropic curvatures (away from solid bodies) in certain organs with the same certainty that I produced heliotropic curvatures. Certain organs, when not in contact with solid bodies, attain, within the same period of time and under otherwise similar conditions, only one-tenth the length which they attain when in contact on one side with a solid body. It is for these reasons that I have made no mistake and performed no useless task in calling attention to the importance of this contact-irritability in the animal kingdom, to which I have found it necessary to give a special name. 3. I have thus far given only examples in which a single source of stimulation determines the “voluntary” movements of animals. Butin a large number of cases the movements of animals are not dependent upon one cause of stimulation 1Dewrtz, Pfliigers Archiv, Vol. XXXVII. See also MAssaArtT, Bulletin de P Académie royale de Belgique (Bruxelles, 1888). Digitized by Microsoft® 112 STUDIES IN GENERAL PHYSIOLOGY alone; more frequently several causes co-operate, and the movements which are produced in this way, the cause of which is again sought by many in the “will” of the animal, are only the resultant of various causes operating at the same time. In very intense light the full-grown larvae of Musca vomitoria move away from the light in the direction of the light rays: they pass by a piece of meat lying in their way. If the light is sufficiently weak, however, the chemical influ- ence of the volatile substances arising from the meat ex- ceeds the orienting influences of the light and the larve crawl to the meat. With other animals which are still more sus- ceptible to chemical stimuli—as, for example, the male Lepidoptera, which, as is well known, are attracted to the female from great distances entirely through the effect of chemical stimuli—heliotropism may be entirely masked by these chemical stimuli. It is not always an easy matter to say, from the movements which an animal always executes, what are the conditions determining these movements. 4. Another complicating circumstance is still to be added. Life-phenomena are phenomena of irritability; 7. e., they are not dependent solely upon the external causes acting upon the organism at a given moment, but upon these and the conditions present within the organism taken together; and the latter conditions are in themselves variable. The study of animal heliotropism revealed the fact that one and the same animal may react differently toward the light during different periods of its existence. The caterpillars of Por- thesia chrysorrhcea after having fasted through the winter are energetically positively heliotropic. After the animals have eaten, heliotropism may still exist, but intense light, which formerly determined their movements with definiteness, has no more effect upon them than did quite weak light previously. Plant lice become sensitive to light—that is to say, positively heliotropic—only after they have fed; the Digitized by Microsoft® On INSTINCT AND WILL IN ANIMALS 113 ‘larvee of Musca vomitoria are energetically negatively helio- tropic only when fully grown, ete. In ants sensitiveness to light is, as I have shown, connected with sexuality. The males are more sensitive than the females; at the time of the nuptial flight the males and females become energetically heliotropic, while the so-called workers remain practically uninfluenced by the light. There must also be mentioned the change which occurs in the sense of heliotropism of many animals in different stages of their development. The full- grown larva of Musca vomitoria is negatively heliotropic; yet the sexually mature insect is positively heliotropic. Such a behavior is quite widely distributed. Finally, it is not infrequently possible to change at will, through the influence of light, positively heliotropic animals to negatively heliotropic animals, and the reverse. The larvee of Balanus perforatus, the larvee of certain worms, and indeed a large number of other animals, become positively heliotropic when they are left in the dark for a long time. If they are brought into light of sufficient intensity, they become negatively heliotropic after a time, and this the more quickly the more intense the light. We do not, therefore, always meet with simple conditions in analyzing the causes which determine the ‘“‘voluntary” movements of an animal; but, however complicated they may be, the ‘voluntary’? movements of animals are never- theless, as our experience indicates, always unequivocally determined only by such circumstances as determine also the movements of bodies in inanimate nature. 5. To be sure, many of the authors who oppose my con-~ clusions would protest if it were said of them that they hold the “will” to be something which cannot be explained on physical or chemical grounds. But if some physical agency is pointed out which prescribes unequivocally the orientation of an animal body or the direction of its movements, which Digitized by Microsoft® 114 STUDIES IN GENERAL PHYSIOLOGY were formerly believed to be determined by the “will” of the animal, the authors are still dissatisfied. They did not doubt that ultimately a physical solution of the question would be found, but they expected something more sublime, something which is more closely related to the mysticism of the ganglion cells. Of course, our knowledge of the process is not exhausted when it is proved that the direction of the rays of light prescribes the direction of the progressive move- ments of Hamatococcus swarm-spores or the Nauplii of Balanus; just as little as the knowledge of the chemical effects of light is today exhausted. Yet no one will say that “instinct” is the determining circumstance in these physical phenomena. 6. Just as the past generation of physiologists felt it to be a handicap that instead of looking for the causes of life-phenomena, investigators were satisfied with the phrase, “The vital force is the cause,” so it is a handicap to us that within the more limited sphere of the so-called psychic life- phenomena the influence of this scholastic method of think- ing has survived to the present time. The handicap lies in the fact, that if one says that “instinct” or “will” deter- mines a motion, the true problem involved is ignored or concealed. This true problem is the analysis of the circum- stances which in each case detérmine unequivocally the “voluntary”? movements of an animal. It was the object of this paper to point out that we must endeavor to solve this problem with as little concern for ‘‘instinct” and “will” as for ‘vital force.” Digitized by Microsoft® IV HETEROMORPHOSIS! I. INTRODUCTION Ir is well known that a number of animals possess the power of forming a new organ in the place of an organ which has been lost. It has always been taken as a matter of course in animal physiology that the regenerated organ is neces- sarily identical in form and function with the one which has been lost. The experience of the botanists, however, shows that this does not hold true in the case of plants, and a few sporadic observations upon animals—which, however, have not been taken into consideration for this problem—seemed to suggest that similar conditions might be found in animals. I have undertaken the task of finding out whether and by what means it is possible in animals to produce at will in the place of a lost organ a typically different one—differ- ent not only in form, but also in function. It is my purpose to report the results of these experiments in the following pages. The organs which I tried to substitute for each other in these experiments are the oral and aboral poles (head and foot). I have succeeded in finding animals in which it is possible to produce at desire a head in place of a foot at the aboral end, without injuring the vitality of the animal. The animal shown in Fig. 16, a Tubularian, has by artificial means been so altered that it terminates in a head at both its oral and aboral ends. If, for any reason, it were necessary to create any number of such bioral Tubularians, this demand could be satisfied. In another Hydroid, Aglaophenia pluma, 1 Warzburg, 1891. The pamphlet is dated 1391, although it appeared in 189, 115 Digitized by Microsoft® 116 STUDIES IN GENERAL PHYSIOLOGY it is possible so to change the form of the animal that it ter- minates at both ends either in oral (Fig. 17) or aboral poles, and yet continues to live. On the other hand, I have found animals in which all attempts at the transformation of organs have thus far been unsuccessful. To this group belong Cerianthus membranaceus and many other Actinians. I succeeded, however, in bringing about a permanent change of form in one of these animals (Cerianthus membranaceus), in which I was able to cause the growth of any number (within certain limits) of mouths, one above the other, in one and the same animal. The regeneration of lost organs in animals has often been made the subject of study, usually, however, only to see which organs can be regenerated, and further to study more closely the anatomical or histological details of the process of regeneration. But it has rarely been considered that these phenomena can give us an insight into the conditions that control the morphogenesis of animals. Where this has been done, it has almost always been with the intention of showing that under all conditions only one and the same organ grows from any definite point on the animal. Allman" was perhaps the first to define this sharply asa law of the formation of organs. From the well-known experiments of Trembley,’ Dalyell,* and from his own obser- vations, he formulated the theory of the “polarity” of the animal body. Allman cut pieces from the stem of Tubu- larians and marked the end which had been directed toward the head of the animal. Even though this cut end was mor- phologically entirely similar to the other, yet a head was formed only at this oral end, while no head was formed at 1GrorcE J. ALLMAN, Report of the British Association for the Advancement of Science, 1864. 2A, TREMBLEY, Mémoires pour servir d Uhistoire dun genre de polypes d'eau douce a bras en forme de cones (Leide, 1744). 3J.G@. DALYELL, Rare and Remarkable Animals of Scotland (London, 1847). Digitized by Microsoft® HETEROMORPHOSIS 117 the opposite end. That Allman chose the name “polarity” for this behavior suggests the possibility that he may have thought of the analogy of this fact to the behavior of a mag- net; for a fragment of a broken magnet always has a north pole at that end which in the original magnet was directed toward the North Pole. If however, the book of Dalyell is subjected to a close scrutiny, it is found that this author occasionally (at two or three places in the book) mentions observations which do not harmonize with the theory of polarity. In these cases, however, Dalyell believed that he was dealing with accidental monstrosities which this careful and patient observer did not consider of sufficient impor- tance to follow out experimentally, or to take into considera- tion for a theory of organization. W. Marshall’ builds on Allman’s theory of polarity in his experiments upon Hydra. When Hydra vulgaris is cut into pieces, ‘‘one is struck most forcibly with the extraordi- nary polarity of the animal, in consequence of which new tentacles and a new mouth are always formed at the oral edge of the cut piece” (p. 698). A further expression of this idea is found in Nussbaum’s papers on “The Divisibility of Living Matter.”’ Nussbaum found that when a piece is cut from an Infusorian, new cilia develop from the edge of the wound in the same number and in the same position that they occupied before the injury. He goes even farther than Allman and concludes that Every minute particle of living protoplasm is oriented; otherwise we could not understand the regular appearance of new cilia at definite points when the infusorian has been divided. Just as we can distinguish in an infusorian between the anterior and the pos- terior, right and left, and dorsal and ventral surfaces, so each minute particle of protoplasm must likewise be oriented according to the three axes in space. 1W. MARSHALL, Zeitschrift fiir wissenschaftliche Zoologie, Vol. XXXVITI (1882). 2M. Nusspaum, Archiv fir mikroscopische Anatomie, Vols. XXVI and XXIX. Digitized by Microsoft® 118 STUDIES IN GENERAL PHYSIOLOGY Almost all the numerous other authors who have worked upon the regeneration of organs in animals also regard it as self-evident that the regenerated organ’ must be identical with the lost organ in form and function. The facts which I shall bring forward in the following pages will show, how- ever, that this theory is certainly too narrow. For I suc- ceeded in doing away with “polarity” first of all in that very animal upon which Allman based his theory of ‘“‘polarity’’—namely, in Tubularia. One of the first authors who concerned himself with the study of the phenomena of regeneration, Charles Bonnet, looked upon them in a less biased way than did Allman. Bonnet, to whom Trembley had very early communicated the fact of the phenomenal regenerating power in Hydra, attempted to convince himself of the truth of Trembley’s statements; since, however, he was unable to obtain Hydra, he tried whether similar results could not be obtained upon worms. Bonnet used two species of worms in his experi- ments. In the first species, which he designates as vers rougeatres, he found the conditions which are typical for Hydra, and which correspond to the theory of “polarity.” If the head of such a worm was cut off, a new head was formed at the cut end; when the tail was cut off, a new tail was formed at the point of section. If the head and tail were both cut off, a head was formed at the oral end, and a tail at the aboral end. In a second species, the vers blanchatres, the results were not so regular. When only the head or tail was cut off, the lost part was always regenerated. If, however, a piece was cut out of the middle of the worm, it happened that such a piece formed a tail at the oral end, instead of ahead. Bonnet observed this three times.’ I have found no reference in the literature which would indicate that these observations of Bonnet have ever been 1CH. BONNET, Guvres d'histoire naturelle et de philosophie (Neuchatel, 1779), Vol. I (Traité d’insectologie). Digitized by Microsoft® HETEROMORPHOSIS 119 repeated and confirmed. I am not in a position to state whether they are correct or not. The theory given by Bonnet is in some points similar to a theory brought forward by Duhamel in his Physique des arbres, and to which Sachs goes back in his papers on “Stoff und Form der Pflanzenorgane.”’' Bonnet believes that just as there are specific germs for the development of the entire animal, there are also special germs for the development of the various organs; he assumes the existence of certain head germs and certain tail germs. In order, however, that these germs may develop, they must be particularly well nourished. Their nutrition is accom- plished, as in plants (according to Duhamel), by various kinds of saps, one of which serves for the nutrition of the head, while the other nourishes the tail. The latter flows from head to tail, the former in the reverse direction. If, now, the head is cut off, the saps which heretofore served to nourish the head, can now be utilized for the nutrition of the head germs, and the latter begin to grow out at the cut oral end into a new head. In a similar way the tail germs may begin to grow when the tail is cut off. It is assumed that the tail germs and the head germs are distributed evenly throughout the body of the vers rougedtres; for this reason a head must always grow from the oral end of a fragment cut from any portion of the animal, while the aboral end must always give rise to a tail. Upon the other hand, in the vers blanchatres the head germs are found only in the neighborhood of the head, while the tail germs are distributed through the entire body. For this reason the worm regenerates a new head when the head is cut off, while a new tail is formed at either end when a piece is cut out of the middle of the worm.’ 1Arbeiten des botanischen Instituts in Wurzburg, herausgegeben von SACHS, Vol. IT (1882), pp. 452 and 689, 2Cn. Bonnet, Considération sur les corps organisés, Art. 259 ff.; Gouvres (Neu- chatel, 1779), Vol. VI, pp. 48 ff. Digitized by Microsoft® 120 STUDIES IN GENERAL PHYSIOLOGY I shall not discuss the importance of the theory of Bonnet. I only mention it here because it takes into consideration the fact that sometimes a tail may be formed instead of a head, which is not done in Allman’s theory of polarity. J] shall avoid all theoretical discussions in this paper, and con- fine myself to the task of showing whether and how it is possible to cause with certainty in an animal the growth of an aboral pole in the place of an oral one, and vice versa, at will. For the formation of an organ which in form and function is different from that which has been lost I shall use the term heteromorphosis. By the term regeneration I under- stand the replacement of a lost organ by one which is identical with that which has been lost. II. HETEROMORPHOSIS IN TUBULARIA MESEMBRYANTHEMUM A layman would be in doubt as to whether he should call a specimen of Tubularia mesembryanthemum a plant or an animal, From a much-branched system of roots (or stolons), which are attached to a solid substratum, arise numerous delicate unbranched stems, several centimeters high, which end in polyps that are usually red and look very much like flowers. These polyps take up and digest the food for the animal. The animals belong to the class of Hydroids and are found in great numbers in the Bay of Naples. The zodlogists have developed a very complicated ter- minology for the individual organs of the Hydroids, which may be very useful in purely descriptive morphology, but does not take into consideration the forms of irritability of the various organs. Causal morphology, which attempts to discover the circumstances that determine form, has to con- sider first of all the irritabilities of the individual organs. For the purposes of the physiologist it is therefore necessary to take these into account in describing and naming the various organs. Digitized by Microsoft® HETEROMORPHOSIS 121 I distinguish in Tubularia, according to the differences in irritability, between the stems and the root. By the root is understood that part of the Tubularian which is endowed with a special contact-irritability (stereotropism), by virtue of which it attaches itself to solid bodies and keeps the animal in a fixed position. By the stem is understood that part of the animal which bears the sexual elements and the polyps, and which is endowed with the opposite irritability, in consequence of which it grows away from the substratum to which the animal is attached. This simple terminology, which is based upon the irritability of the organs, will suffice for our purposes. Of the entire animal only the polyps can move spontaneously; the stem is immovable. If we cut a piece out of a stem, we must discriminate between its oral and aboral ends, according to the orientation of the piece in the original uninjured animal. The oral end is that which was originally directed toward the polyps, the aboral end, that which was directed toward the root. I shall now describe the main experiments individually. 1. I cut off the roots and polyps of a series of stems, and put these mutilated stems with their aboral ends ver- tically into the sand sufficiently deep to keep them in a ver- tical position. At the free oral ends, which were surrounded on all sides by sea-water, new polyps were formed in a short time—at the proper temperature and with favorable speci- mens within two days. These corresponded in form with the old polyps. No growth took place at the ends which were buried in the sand, no matter how long the observations were carried on (in some instances for several months). When I put stems with their oral ends in the sand, a polyp was formed at the free, aboral pole. In favorable cases this was formed in a few days. Neither a polyp nora root was formed at the oral end, which had been covered by sand, no matter how long I continued my observations. Digitized by Microsoft® 122 STUDIES IN GENERAL PHYSIOLOGY Contrary to the theory of the “polarity” of the animal body, therefore, fragments of Tubularia mesembryanthemum are able to form polyps even at their aboral ends. 2. I supported pieces cut from the stem of Tubularia mesembryanthemum in such a way that both cut ends were surrounded by water. To do this I sup- ported them in the meshes of a long wire net, or in the holes of a metal plate set up in the aquarium for this purpose. Polyps were formed at both the oral and the aboral ends of the fragments, so that the stem terminated in a head at each end. Fig. 16 represents such an animal sketched from life and enlarged twice. ab is the piece removed from the old Tubularian. Polyps were formed at both ends, and the stem then grew in length from both ends. ac and bd are the new pieces that grew after the formation of the polyps. I have in this way been able to produce at any time any number of animals which terminate in an oral pole at each of the two ends of their body. I shall hereafter designate animals which terminate in a head at each end bioral animals. I would particularly emphasize the fact that such an animal remains bioral for the rest of its life. It is a well-known fact that in a normal animal the oral polyp is lost spontaneously after some time, and that a new one is formed sooner or later in its place. In the case of the bioral animals a constant blooming, shedding, and reappearance of the polyps occurs, not only at the oral end, but also at the aboral end, during the entire duration of their life. 3. I was able, therefore, not only to cause the develop- "FIG. 16 Digitized by Microsoft® HETEROMORPHOSIS 123 ment of a head at both ends of the fragment of a stem, but also to prevent the formation of a head at will, by simply putting this pole into the sand. When both poles are put in the sand, no head is formed at either end. If one of the poles which has been in the sand for some time, and on which the formation of a head has been prevented in this way, is pulled out of the sand so that it is again surrounded on all sides by water, a head may form at this end. If the animal is covered only by an exceedingly thin layer of sand, a polyp will still be formed which makes its appearance between the grains of sand, much as the stem of a ger- minating seed may grow through a thin layer of earth. One of the poles of a piece of a stem was pushed between two slides laid upon each other and held together by thin rubber bands. Needles were placed between the two slides, and one end of the stem of the Tubularian was laid in the wedge-shaped space thus formed. In this way the end was subjected to slight pressure. No polyp was formed at the end subjected to this slight pressure, no matter how long I waited; while at the other end, which was not pressed upon and was surrounded by water, a polyp was formed in the usual time. When the piece was removed from between the slides, a new polyp frequently developed at the end that had been subjected to the pressure. That light is not necessary to the formation of a polyp was proved by the fact that pieces of Tubularian stem will grow new polyps in a dark- ened vessel. The experiments described were made in well- aérated aquaria. 4. When the polyps and the roots are cut off from long stems of Tubularia mesembryanthemum, it is found that the new polyps are always formed one, two, or three days earlier at the oral than at the aboral end. I believe that the cause of this phenomenon, which may be considered as an intima- tion of “‘polarity,” lies in the fact that when long pieces are Digitized by Microsoft® 124 STUDIES IN GENERAL PHYSIOLOGY cut from the stem, the lumen at the cut oral end is usually wider than that at the aboral end; for when I cut from the middle of the stem shorter pieces, which showed no differ- ence in the diameter of the lumina, a polyp often formed earlier at the aboral end than at the oral. 5. The size of the newly formed polyp also depends to a certain extent upon the diameter of the stem at the cut end. When the diameter was very small, the polyp was also very small; when the diameter was large, the polyp was also larger. 6. It might still be imagined that, besides the mechani- cal factors thus far considered, a physiological factor might also play ardle. It might be thought that the substance of which the polyp is formed is present in a larger amount at the oral than at the aboral pole. To test this point I chose a large number of very long Tubularian stems that had been cut off close to the roots, and at the cut ends of which polyps had been grown. I bisected these stems transversely, and kept the oral and aboral halves in separate beakers. If the substance required for the formation of the polyps were unequally distributed in the stem, then the one series of fragments should have formed polyps sooner than the other series. This was never the case; but—as was again noted —every fragment formed a polyp sooner at its oral than at its aboral end, even though the difference in time often amounted to only one half-day or less. 7. While I have always succeeded—with suitable mate- rial, and with the experiment under the proper external con- ditions—in making a head grow at the aboral end of the stem, I have thus far not yet succeeded in making a root grow at the oral end of a stem. When I cut off the stems close to the substratum to which the roots were attached and brought the aboral ends in contact with the walls of the aquarium, the end, when it grew at all, attached itself to the Digitized by Microsoft® HETEROMORPHOSIS 125 solid body and became a root; however, when contact with the wall of the aquarium was broken so that water sur- rounded the root on all sides, a polyp was formed also at the end of the root. In my further experiments I shall try to find conditions under which the animal will form roots at both poles with just as great certainty as it now forms heads. From the experiments thus far discussed, I can only con- clude that the formation of polyps in Tubularia mesembry- anthemum can be brought about much more easily than the formation of roots. Ill THE LIFE-PHENOMENA OF THE ORAL POLE OF TUBU- LARIA MESEMBRYANTHEMUM Doubt might arise as to whether the two heads of a bioral Tubularian manifest the same life-phenomena; as to whether the two morphologically equal poles are also identical physio- logically. I shall show that this is, indeed, the case, and in doing so shall dwell a little more upon the differences in the irritability of stem and root. 1. The stem and root of Tubularia mesembryanthemum have an entirely different contact-irritability. If the root is brought in contact with a solid body, it attaches itself to it, and in its further growth remains closely attached to the sur- face of the solid. If an attempt is made to lift the stem from the solid body, it tears off close to the root, the latter remaining attached to the base upon which it grew. The polyp has exactly the opposite irritability. When the polyp comes in contact with a solid body—for example, when the stem lies horizontally upon the bottom of the aquarium —it soon grows away from it. The growing region of the stem (which is situated close behind the polyp) becomes convex against the solid substratum. This (stereotropic) bending occurs only in the growing part of the stem, and persists when growth has ceased, just Digitized by Microsoft® 126 STUDIES IN GENERAL PHYSIOLOGY as do geotropic or heliotropic curvatures in many growing plants. To bring about this stereotropic curvature it is necessary that the polyp itself should come in contact with the solid body. If any part of the stem alone comes in con- tact with the solid, no bending occurs, even though the growing part of the stem, close to the polyp, touches the solid. The contact-irritability of the polyp is opposite in kind to that of the stem; the stem is positively, the polyp is negatively, stereotropic. The negative stereotropism of the polyp may be clearly demonstrated in the following simple manner: Beheaded Tubularians were fixed in a beaker half-filled with sand in such a way that one end was fixed in the sand, while the other end just touched the side of the vessel. As soon as the new polyps were formed and the Hydroids began to grow in length, the tips of all the stems bent away from the glass sides of the vessel. The direction of the rays of light had no effect upon this process. In all these experiments the polyps formed at the aboral end behaved exactly like those formed at the oral end. 2. I have not succeeded in bringing about either helio- tropic or geotropic curvatures in Tubularia mesembryanthe- mum. When I fastened the stem in the middle, and when both ends were surrounded by sea-water on all sides, the stem of the bioral Tubularian continued to grow in the direction of the old piece; it mattered not whether it lay in a vertical or in a horizontal position, or in which direction the light struck it. This is a remarkable fact, for, in looking at a colony of Tubularians, one might easily be led to think that they possess heliotropic or geotropic irritability, as the stems of such a colony upon the surface of a solid are all arranged in the same way. Yet the similarity in the orientation might be determined in the main by their contact-irritability. The oral ends of the young stems Digitized by Microsoft® HETEROMORPHOSIS 127 grow almost perpendicularly away from their substratum. If, in addition, the separate stems stand very close together, as is usually the case, the contact of the polyps with each other influences their orientation. This has the same effect as would be brought about by causing each separate polyp to grow in a narrow hollow cylinder. The individual stems must thus not only grow away from the surfaces to which they are attached, but they must grow away from it in approximately straight lines. 3. Dalyell has observed in Tubularia indivisa—a form very similar to Tubularia mesembryanthemum—that the polyps drop off after they have existed a certain length of time, and that after a longer or shorter period new polyps are formed in their places. As soon as a new polyp has been formed, the stem begins to grow in length immediately under it. The growth continues as long as the polyp exists; as soon as it drops off, growth ceases.’ I observed the same condition of growth in Tubularia mesembryanthemum. The longitudinal growth of the stem was continued to a region just beneath the polyp, and it continued as long as the polyp existed; when the latter dropped off, growth ceased; when a new polyp was formed, the stem again grew in length. In the bioral polyps an increase in length occurred simultaneously at both ends of the stem, so that these stems reached a much greater length in a shorter time than any of the normal specimens that were ever brought to me by the collectors of the Zodlogical Station in Naples. That the stem grows in length close behind the polyps at both ends of the bioral animal is clearly shown by the fact that the newly formed part is thin and transparent, and thus can be readily distinguished from the older opaque portions of the stem. Therefore in its growth also the aboral pole of Tubularia behaves like the oral. 1 Rare and Remarkable Animals of Scotland (London, 1847). Digitized by Microsoft® 128 STUDIES IN GENERAL PHYSIOLOGY 4, I did not succeed in observing the polyps in the process of taking up food. Yet I have noticed in both the oral and the aboral polyps the same sudden closure of the tentacles which occurs in Actinians when they seize their food and swallow it. I shall show later in Actinians that heads which have been newly formed in abnormal places behave like normal heads in the matter of taking up food. IV. THE DISCREPANCY BETWEEN ALLMAN’S THEORY OF PO- LARITY AND THE BEHAVIOR OF TUBULARIA MESEM- BRYANTHEMUM 1. I mentioned in the introduction that Allman based his theory of polarity on observations made upon Tubularia. The discrepancy between Allman’s ideas and my observa- tions compels me to enter into a more detailed discussion of his theory. The passage in Allman’s treatise which is of interest to us is the following: There is thus manifested in the formative force of the Tubu- laria stem a well-marked polarity, which is rendered very apparent if a segment be cut out from the center cf the stem. In this case, no matter in what position the segment may be, that end of it which was directed downward or proximally, while it formed a part of the unmutilated hydroid, will never develop a polypite, but will extend itself as a simple prolongation of the coenosare; while the upper or distal end, instead of becoming simply elongated, will shape itself into a true polypite; and all this is true, though of course not the least difference in structure or form can be detected between the two extremities at the time of section.! Allman adds in a note that the observations of Dalyell, who made numerous regeneration experiments upon Tubu- laria indivisa, are in perfect accord with his own. By reading Dalyell’s paper one, indeed, finds the same idea expressed as by Allman, although the term “polarity” is not used. It might be thought that Tubularia indivisa, upon 1Loc. cit., pp. 392 ff. Digitized by Microsoft® HETEROMORPHOSIS 129 which Allman and Dalyell experimented, behaves typically differently from Tubularia mesembryanthemum, upon which I made my experiments. bottle B. In} oO : this way the effect vf pure oxygen + : could be compared with that of atmos- a on a pheric air. A few important but self- evident details in the arrangement of the experiment have been omitted in the drawing. In one experiment eggs which had been in the eight-cell stage, but the cleavage-cells of which had been fused by ex- Digitized by Microsoft® 396 STUDIES IN GENERAL PHYSIOLOGY posure to a current of hydrogen, were introduced into both gas-chambers. I wished to determine whether the renewal of segmentation would occur more rapidly and differently in pure oxygen than in air. The result was that after fifty minutes cleavage occurred almost simultaneously in both gas-chambers and in exactly the same way. Cleavage occurred only at the periphery and the cells which were formed were about the size of those found in the thirty- two- or sixty-four-cell stage. Ina second experi- ment cleavage occurred even a little more rapidly in the air than in pure oxy- gen. For the rest things were about the same. Under these circumstances I saw no reason for con- tinuing these experiments ; r they showedclearlyenough 4 that it does not —— matter, so far B as the renewal of cleavage of A liquefied Cten- olabrus eggs is concerned, whether air or pure oxygen is FIG. 123 Digitized by Microsoft® PHYSIOLOGICAL Errects oF Lack oF OxyGEen 397 supplied to them. We found before that a lack of oxygen does not retard cleavage as long as cleavage is at all possible. In the same way an excess of oxygen does not accelerate the process. VIII. EFFECT OF LACK OF OXYGEN ON THE CLEAVAGE OF THE FUNDULUS EGG The eggs of Ctenolabrus have a lower specific gravity than sea-water, and therefore float at the surface of the water. Here they find the oxygen necessary for their devel- opment. If the eggs of Ctenolabrus with their great need for oxygen had a specific gravity large enough to cause them to sink to the bottom, they could scarcely develop in many places, since at the bottom of the ocean where processes of putrefaction are going on, the tension of oxygen is much less than at the surface. We may therefore expect, in gen- eral, that fish eggs which sink to the bottom of the ocean and develop there are much more independent of oxygen than the egg of Ctenolabrus. This is really often the case. The egg of Fundulus has a greater specific gravity than sea- water and develops at the bottom of the ocean. I have shown that the egg of Fundulus can develop for some time in the absence of oxygen. In these experiments the eggs were introduced with a few drops of sea-water into a small glass tube sealed at its lower end, and this tube was put into a test-tube containing several cubic centimeters of an alkaline pyrogallol solution. The test-tube then was sealed at the top. The pyrogallol solution was prepared according to Hempel’s directions, and the oxygen must have been ab- sorbed in a short time. Nevertheless, the eggs not only segmented, but they developed as far as normal eggs do in about fifteen hours after fertilization. A large blastoderm was formed which spread over a great part of the surface of the egg. Digitized by Microsoft® 898 STUDIES IN GENERAL PHYSIOLOGY In order to be able to compare these results with those obtained on the Ctenolabrus egg, I repeated the experiments on Fundulus, using the same method of replacing oxygen by hydrogen, and the same apparatus which had been used in the case of the Ctenolabrus egg. The results obtained were in entire harmony with our earlier findings. When freshly fertilized eggs of Fundulus are introduced into the Engelmann chamber, and a vigorous stream of hydrogen is passed through it, the eggs divide not only once, but continue to do so for fifteen to twenty hours, until a blastoderm is formed which extends over the sur- face of the egg. The result was the same when the’ eggs were put in an Engelmann chamber and kept for two and one-half or three hours on ice, during which time they were exposed to a vigorous stream of hydrogen. When the eggs were then exposed to room temperature, segmentation at once began and continued in a regular manner. During the entire course of the experiment hydrogen was permitted to pass through the chamber. As long as the number of the cleavage-cells was so small that they could be counted, it could be seen that develop- ment without oxygen occurred as rapidly as in oxygen. Whether this holds also for later stages when cleavage approaches the standstill cannot be determined, as the cells are then too small to allow one to count them. Not only cleavage, but also growth, of the blastoderm, that is to say, increase in area (at the expense of the yolk (?), 1903) occurs in the absence of oxygen. The blastoderm grows from a small area to a large area on the surface of the yolk. If Fundulus eggs are allowed to remain more than twelve to fifteen hours in hydrogen, the cells nevertheless do not liquefy, as is the case in Ctenolabrus in the absence of oxygen. Even after twenty-four hours no such phenomena are observable in the Fundulus egg. I have shown in Digitized by Microsoft® PHYSIOLOGICAL EFFEOTS oF Lack oF OxyGENn 399 previous papers that such eggs do not lose their power of dividing even after being kept for three to four days with- out oxygen. On the other hand, I noticed a collection of the strongly refractive droplets in the furrows between the cells in Fundulus eggs also. Our observations on the mechanics of cell-division, therefore, seem to hold also for the Fundulus egg, only that the material for the surface layer of the Fundulus cells seems to be different chemically from that of the Ctenolabrus cells in that the latter in the lack of oxygen flows together into droplets, while the former undergoes no such structural changes. On the other hand, the Fundulus egg is very sen- sitive to carbon dioxide. If a current of carbon dioxide is passed through the gas-chamber in which are contained the freshly fertilized Fundulus eggs, not a single cleavage occurs. Furthermore, the eggs which have resided for only four hours in such a current of carbon dioxide have lost their power of development for all time. This is of great impor- tance in judging of the effects of lack of oxygen—it points to the possibility that the resistance of the protoplasm to lack of oxygen is not so very different in the Ctenolabrus egg from that in the Fundulus egg, and that only a second- ary molecular change—the disintegration of the surface layer of the cells into a number of droplets—brings about a rapid destruction of the Ctenolabrus cells. This possibility is supported by another fact. I have pointed out in an article, which I have already cited, the remarkable indifference of the Fundulus egg to the concen- tration of the sea-water. This year Professor W. W. Nor- man made similar experiments in my laboratory upon the Ctenolabrus egg. In these it was found that the Ctenolabrus egg is almost as insensitive to an increase in the concentra- tion of the sea-water as is the Fundulus egg. T should not like to conclude this section without adding Digitized by Microsoft® 400 STUDIES IN GENERAL PHYSIOLOGY a word on the importance of comparative methods in physi- ology. If we had confined our experiments to the Cteno- labrus egg, a generalization of the facts observed would have been as follows: Cleavage is impossible without oxygen. Had we confined our experiments to the Fundulus egg, we should have come to the opposite conclusion. In reality, conditions are such that in some forms a cleavage is possible without oxygen, while in others it is impossible. The same may be said regarding protoplasmic motion. I do not as yet consider it as settled that every muscle is able to do a large amount of work without free oxygen. IX. THE EFFECT OF THE REMOVAL OF OXYGEN ON THE SEGMENTATION OF SEA-URCHIN EGGS If freshly fertilized sea-urchin eggs are introduced into a gas-chamber and a strong current of hydrogen is sent through it, one cleavage always occurs, and sometimes two. If, however, before beginning the actual experiment, all of the oxygen necessary for cleavage is driven out of the eggs and the gas-chamber (by placing the latter upon ice for two hours and sending a current of hydrogen through it), no cleavage occurs, even though we wait from three to four hours. If after this the eggs are again exposed to air, cleavage begins in about forty to fifty minutes. But all the eggs first divide into two cells, and only a few divide at once into three or four cells. The number of the latter is not greater in the experimental eggs than in the normal eggs of the same culture. Such phenomena are very probably attributable to polyspermia. These facts show that in sea- urchin eggs neither a division of the cell nor of the nucleus is possible without oxygen. In this particular they behave like the eggs of Ctenolabrus. We must now raise the ques- tion: Is the inability of cleavage in sea-urchin eggs also the consequence of molecular changes which are brought about by lack of oxygen? This, indeed, seems to be the case. Digitized by Microsoft® PHYSIOLOGICAL EFFEcTs oF Lack oF Oxygen 401 If the eggs have divided into two or four cells, and the oxygen is then removed completely from them, the cell-limits become indistinct in about three hours. The cells then absorb water in consequence of the effects of lack of oxygen. The volume of the eggs increases, and the space within the membrane is soon filled uniformly with the protoplasm of the cleavage-cells. The outlines of the cell then become invisible, and the egg looks as if it had never divided. If oxygen is readmitted, the eggs cleave anew, if too long a time is not allowed to elapse. In many cases the old lines of cleavage reappear, but this is by no means always the case. The changes remind one of those in the eggs of Ctenola- brus, only that they occur more rapidly and more distinctly in the latter than in the eggs of the sea-urchin. The surface of the cleavage-cells of the Arbacia is pig- mented, and the pigment granules move upon the surface of the egg during cleavage. I do not doubt that by more care- ful study phenomena similar to those observed in the cleavage of the Ctenolabrus and the Fundulus eggs will be observed in the case of Arbacia also. The fact has been mentioned that, in general, the cleavage of the Fundulus egg without oxygen occurs not only just as rapidly as under normal conditions, but even a little more rapidly, as stated in my paper on “The Relative Sensitiveness of the Fundulus Embryos in the Different Stages of Devel- opment against Lack of Oxygen.” In that article, however, I attributed this difference in time to the increase in tempera- ture brought about in sealing up the test-tubes used in the experiments. Since I again noticed these changes this year, first in the Ctenolabrus egg, and later in the Arbacia egg, in an Engelmann chamber—where there was, therefore, no considerable increase in heat——I decided to determine by more careful experiments whether this difference in time is indeed dependent entirely upon differences in temperature, Digitized by Microsoft® 402 STUDIES IN GENERAL PHYSIOLOGY or whether the altered metabolism in the initial lack of oxygen does not at first lead to a slight acceleration of cleavage. Ifthe latter were correct, it would give a basis for the explanation of a very purposeful arrangement in organic nature, namely, the increase in respiratory activity in the lack of oxygen. For if lack of oxygen leads to such a universal change in metabolism that more energy is at first set free than under normal conditions, then the purposeful arrangement of the respiratory center is only a special case of a general property of protoplasm. Yet the acceleration of cleavage in the Engelmann chamber might also be dependent upon an increase in temperature. One source of this increase in temperature might be sought in these experiments in the heat produced in developing hydrogen from zine and sulphuric acid. The gas was passed through four wash-bottles before reach- ing the gas-chamber, yet it might nevertheless have caused an increase in the temperature in the gas-chamber. To render this impossible or less possible the gas generator was packed in a vessel with ice before beginning the experiment. From this the hydrogen was led through a bottle filled with chipped ice which was in turn again packed in ice. The first three wash-bottles were also kept on ice. The temperature of the last wash-bottle through which the gas passed before reaching the gas-chamber was carefully watched before and during the experiment. No increase in temperature was noted when the hydrogen was passed through it. The same water was used for the eggs in the gas-chamber that was used for the control eggs. Every decrease in the temperature of the latter through evaporation of the water was carefully avoided, and their temperature carefully watched. Cleavage in the eggs kept in the gas-chamber neverthe- Digitized by Microsoft® PuHyYSsIOLOGIOAL Erreots oF Lack or OxyGen 403 less preceded that in the normal eggs by three or four min- utes. The experimental eggs as well as the control eggs were fertilized at the same time and in the same dish with a large amount of sperm. The process of driving out the oxygen by hydrogen was begun some ten or fifteen minutes after fertilization. About half an hour later cleavage occurred, usually first in the gas-chamber. At this time all the oxygen was probably not yet driven out of the eggs, so that we were dealing only with a partial lack of oxygen. This partial lack of oxygen, therefore, often brought about an acceleration of cleavage equal to 6 to 10 per cent. of the time necessary for the first cleavage.’ These experiments give one the impression that when lack of oxygen has reached a certain stage, a transitory increase in the development of energy occurs within the egg at first (through the formation of poisonous substances?). This increase in the development of energy, which, in the case of the respiratory center, is of enormous practical importance, therefore seems to appear also in such cases where its appearance is entirely unimportant, as in cleavage. I will not yet commit myself definitely to the statement that in case of a partial lack of oxygen a transi- tory acceleration of cleavage occurs; but to trace back the purposefulness of organized nature to the general chem- ical and physical properties of protoplasm seems to me much more promising than the assumption of natural selection. If we summarize the results of these experiments on the effects of lack of oxygen on cleavage, we find that in the Fundulus egg, where in the absence of oxygen no dissolution of the cell-walls of the cleavage-spheres occurs, cleavage can continue for more than ten hours without oxygen; while in 1T am still inclined to believe that, in spite of all the precautions, the hydrogen had a slightly higher temperature than the air when it reached the eggs. [1903] Digitized by Microsoft® 404 STUDIES IN GENERAL PHYSIOLOGY the Ctenolabrus, and eggs’ which cannot cleave without oxygen, the surface layer of the cleavage-cells is liquefied and the cells fuse together. The latter fact seems to indi- cate that cleavage does not occur in certain eggs, because without oxygen profound molecular changes occur, which, among other things, prevent the formation of a membrane or a specific surface film. X. ON THE EFFECT OF LACK OF OXYGEN ON CARDIAC ACTIVITY IN FISH EMBRYOS The older experiments on the effect of lack of oxygen on the activity of the heart have in part led to strange results. Tiedemann, for éxample, found that when the heart of frogs or salamanders is excised and kept under the bell of an air-pump, it ceased to beat in less than one minute when the air is rarified.’ Castell’ came to more probable results. He found that when the heart is cut out of the body of a frog and kept in an indifferent medium in the absence of oxygen, it may continue to beat for an hour. In the experiments of Pfliger and Aubert, which have already been mentioned, the heart continued to beat after all the spontaneous movements of the animal had long ceased. The older authors had discussed the question as to whether oxygen does not have a direct stimulating effect upon the heart. This would, of course, explain why the heart ceases to beat when oxygen is lacking. Castell, however, showed that a heart which has ceased to beat in an atmosphere free from oxygen will also not beat when stimulated by other means. The papers which have been cited in the introduc- tion give a more rational explanation of the rdle of oxygen 1 This phenomenon is less distinct, and therefore not so certain, in the egg of Arba- cia as in that of Ctenolabrus. Driesch questions it in the sea-urchin egg, but I am not certain that his experiments are identical with mine. [1903] 2 Archiv fiir Anatomie und Physiologie, 1847, p. 490. 3 [bid., 1854, p. 226. Digitized by Microsoft® PHYSIOLOGICAL EFrects oF Lack or OxyGen 405 than that furnished by the assumption that the oxygen “stimulates” the heart. I was especially interested in comparing the effects of lack of oxygen on the beat of the heart in Ctenolabrus and Fundulus embryos. Does the same difference in behavior toward lack of oxygen exist here as in regard to cleavage? The heart begins to beat and the circulation is estab- lished in Ctenolabrus embryos as early as forty-eight hours after fertilization. If such forty-eight-hour-old embryos, which are still contained within the eggs, are introducedintoa gas-chamber through which a current of hydrogen is passed, the heart usually comes to a standstill in from three to ten minutes after the current of gas is turned on. The activity of the heart does not, however, gradually fall to zero, but the heart comes to a standstill suddenly when the number of heart-beats has decreased but little or not at all. In one case the heart beat about 90 times a minute before the hydrogen was admitted. Hydrogen was then passed through the gas-chamber, and after four minutes the heart still beat 89 times; two minutes later it beat 78 times, and in the following minute 77; in the next minute the heart came to a sudden standstill. After hydrogen had been passed through the gas-chamber for only seven minutes, and when the number of heart-beats had fallen only from 90 to 77—a slight de- crease only—the heart suddenly stood still; at that time blood was still circulating beautifully. In a second experiment the number of the heart-beats was 108 per minute at the beginning of the experiment. Two minutes after turning on the hydrogen gas the heart beat 105 times, and three minutes later 108 times a minute. During the next minute the heart stood still after having beaten 23 times in the first eighteen seconds of that minute. The heart stood absolutely still for four minutes, after which it gave a few weak pulsations. For the next three minutes Digitized by Microsoft® 406 STUDIES IN GENERAL PHYSIOLOGY it again stood still, after which the heart beat rhythmically for one minute (38 beats in a minute), when it again ceased. A few irregular pulsations followed, and then everything was over. Sixteen minutes after turning on the current of hydrogen the heart had come to a complete standstill, but the embryo itself still moved at this time, and even five minutes after the heart and the circulation had ceased entirely the embryo still moved! In a third experiment the current of hydrogen was turned on at 11:26 a.m. The number of beats was 90 per minute; in the following minute it was 81, and in the third minute the heart came to a sudden and permanent standstill. Ina fourth experiment the current of hydrogen was started at 10:03 a.m. The number of-heart-beats was 100 per minute. The following table indicates the course of the experiment: 10:03 100 beats per minute 10:04 102“ * i 10:05 100 * « 10:06 9G. hs us 10:07 98 ss se 10:08 OO. es 10:11 - 60 “ & ss 10:12 Bas oie me 10:13 54 “ « se The heart then came to a sudden standstill. Three min- utes later the heart again beat twice; shortly after this it beat regularly for one minute (39 times per minute). The heart then again stopped; a few scattered beats followed, and at 10:25 a. m. the heart came to a permanent standstill. When the embryos whose hearts had come to a standstill were returned, after not too long a time, to water containing oxygen, resuscitation of the heart followed, and this the earlier, the shorter the time the embryo had remained in the atmosphere free from oxygen. If the eggs remained for one to one and a half hours in the gas-chamber, they became Digitized by Microsoft® PHYSIOLOGICAL Errreots or Lack or Oxycren 407 opaque and sank to the bottom. Twenty-five minutes after turning on the hydrogen the changes which we have described in detail above—namely, the appearance of the strongly refractive droplets—were often clearly visible. If now we ask for the cause of the rapid and sudden standstill of the heart of Ctenolabrus embryos when deprived of oxygen, we must admit, first of all, that a failure of the energy which is supplied perhaps by processes of oxidation cannot be the cause. For, since the oxygen is replaced by hydrogen only gradually, the number of heart-beats should under these circumstances also decrease only gradually until a minimum is reached. The behavior of the heart was, how- ever, entirely different. The heart usually came to a stand- still without a noteworthy decrease in the number of heart- beats; sometimes a decrease was noted. For the same reasons the view that in three to ten minutes after turning on the current of hydrogen all the potential energy present in the heart has been used up is also to be set aside. After the heart had ceased to beat, the entire animal still executed spontaneous movements, and the heart remained generally active in case of lack of oxygen longer than the rest of the body of an animal.’ The rapid and sudden standstill of the heart of Ctenolabrus is the consequence either of a sudden poisoning, or of a structural change in the heart brought about by the removal of oxygen. It might also be that the poisonous effect consists only in bringing about molecular changes. The experiments on the cleavage of the Ctenola- brus egg showed that a change occurs in the cell-walls in consequence of which they break up into droplets. We must assume that these changes are brought about by the beginning lack of oxygen, or the metabolic products formed in consequence of this lack of oxygen. Might it not be pos- sible that a liquefaction of solid elements and the formation 1 Miss Moore has since found that in young fish whose respiratory and sponta- neous motions have ceased the heart still continues to beat for hours. [1903] Digitized by Microsoft® 408 STUDIES IN GENERAL PHYSIOLOGY of droplets hinders the production or the transmission of molecular movements, and in this way brings about the sud- den standstill of the heart? This idea would also harmonize very well with the fact that the heart comes to a standstill as suddenly and as unexpectedly as death ensues from embol- ism. It would also be in harmony with this idea that after the sudden standstill of the heart a few occasional heart- beats may yet appear. We will, however, not enter farther into the field of hypotheses, but rather attempt to see how the heart of Fundulus behaves in the lack of oxygen. Numerous experiments on embryos from four to ten days old (the embryos do not hatch until after the twelfth day) showed without exception the following behavior of the heart in the case of lack of oxygen: During the first ten to twenty minutes after the hydrogen is turned on through the gas-chamber, the number of heart- beats does not decrease. A transitory acceleration even occurred, which, however, was brought about through a rise in temperature caused by passing the hydrogen gas through the gas-chamber. This acceleration did not occur when I packed the hydrogen generator in ice. But the decrease in the amount of oxygen contained in the Fundulus egg, which occurs during the first twenty minutes and which causes the heart of the Ctenolabrus embryo to stand still, has no effect upon the rate of the heart of the Fundulus embryo. Then follows a period of steady decrease in the number of heart-beats, which continues for about one and one-half hours. The decrease occurred most rapidly at first and then more slowly. During this period the number of heart-beats fell from about 120 or 100 a minute to about 20 per minute. This period corresponds, it seems to me (and we shall find further proofs for this idea later), to the period of progres- sive decrease in the oxygen necessary for the oxidations in the heart. Digitized by Microsoft® PHYSIOLOGICAL Erreots oF Lack ofr Oxyarn 409 When the number of the heart-beats has decreased to the minimum of about 20 per minute, the heart continues to beat at this rate for about eight to ten hours in an uninter- rupted and regular manner, until at the end of this time it comes to astandstill. Since our earlier experiments rendered it possible that after two hours all the exhaust- = eal ible oxygen has certainly been driven out by - the current of hydrogen, we are perhaps justified » 004 in assuming that the energy for this long- & continued and regular, but slow, activity of the [ee heart is derived from processes of hydrolysis. go -| It seems as if we are able in the Fundulus heart to separate numerically the energy derived oe from hydrolytic processes from that derived bo from processes of oxidation, in that the former source of energy yields about 20, the latter the 50 J remaining, about 80 to 100, heart- ee beats per minute. I would especially emphasize the fact that during the a entire time of the experiment the Zo fo 4 ° 1 2 3 ¥ F 6 7 8 7, «&@ 4 4 — Hours FIG. 124 current of hydrogen was passed through the gas-chamber uninterruptedly, and that in consequence every action of the carbon dioxide had been shut out in these experiments, as in those upon the Ctenolabrus embryo. We shall now describe a few of the individual experi- ments. In one case the hydrogen current was turned on at 8:42 a.m. The number of heart-beats was 108 to 114 per Digitized by Microsoft® 410 STUDIES IN GENERAL PHYSIOLOGY minute. This number remained constant until about 9:08. (The current of hydrogen was not as vigorous as usual.) At 9:12 the number of heart-beats was 96; at 9:30 the number was 69; at 10 the number was 48; and at 11 it had fallen to 27. At 11:25 the heart beat 23 times per minute; at 11:40 it beat 20 times per minute; after which the number of beats varied between 20 and 23 per minute, until 8:45 Pp. mM.; in other words, more than nine hours. The curve of Fig. 124 illustrates the condition of affairs better than description. The curve is typical and may be looked upon as representing any one of these experiments. Only the absolute values varied with different individuals and with the temperature. In another experiment the current of hydrogen was turned on at 3:06 a.m. The number of heart-beats was 120. At 3:17 the heart beat 126 times, after which the number decreased, as shown in the following table: 3:20 110 beats per minute 8:22 86(1)“ “ 3:25 60 “ « a 3:27 b4 7 3:31 50 “ « - 3:34 44 “4 . 8:40 386 “ 79 66 8:45 33 “ 3:52 2446 « % 4:00 22, “se* Sse 4:05 20 “ . 4:12 1g ee = 4:20 16“ « ‘ 4:30 14 “ “c 66 4:55 12,08 ON “ This rate continued unchanged until 9:50, when the experiment was brought to a close. It is readily seen how much more rapidly the decrease occurs at first than later. Fig. 125, which illustrates the beginning of this experiment, shows this very strikingly. Digitized by Microsoft® PHYSIOLOGIOAL EFrFrects oF Lack or Oxyaren 411 Ihave repeated this experiment eight times, always with It was of importance now to determine whether the number of heart-beats increases, and how much it increases, when a heart which has attained its minimum rate in hydrogen is again exposed to the oxygen of the air. the same result. 100 = — SJR 74% HY 3 i fo - ie5 bo 4 fo ~ ¥o + 30 + ho ~ fo = In one such experiment the current of hydrogen was turned on at 9:10 a. Mm. The number of heart-beats was 120 per minute. At 11 the number of heart-beats had fallen to 42, and soon thereafter the minimum of 24 heart-beats was reached. At 2:40 the number of heart-beats was still 24. At 2:44 the embryo was taken out of the gas-chamber and brought into fresh water, and at 2:48 the num- ber of heart-beats was counted; it was then 30. The further course of the experi- ment is shown in the following table: 2:48 2:49 2:50 2:55 3:00 3:03 4o so bo qo 80 70 yoo 0 POinu bes FIG. 125 40 beats per minute 5L “ ce “6 60 ory oy “ce 66 ae 66 a“ fe 66 te be ce 69 T9 ae “cc Digitized by Microsoft® 412 STUDIES IN GENERAL PHYSIOLOGY 3:05 - 75 beats per minute 3:08 - SE ee ss 3:15 Ba Se et of 8:25 96 oc 6c “ 3:35 102 “ = * 6 3:47 did sf 3:53 120% © a“ This rate continued until 5:10, when it increased to 132.' This experiment, which I repeated several times with the same result, shows that the decrease in the number of heart- beats when the heart is deprived of oxygen is dependent chiefly upon the decrease in the energy furnished by oxida- tion and not upon the formation of poisonous substances. The fact that the minimum number of heart-beats continues a very long time without oxygen also speaks against the latter idea. We can make use of still another method to determine what proportion of the heart-beats in the Fundulus embryo depends upon oxidations, and what proportion upon pro- cesses of splitting. By placing the gas-chamber upon ice and passing a current of hydrogen through it, we are able to drive out the oxygen, while the processes of hydrolysis are at the same time reduced to a minimum through the lower- ing of the temperature. In one experiment I passed the hydrogen through the gas-chamber for two hours, while keeping it on ice. The hearts were then removed from the ice, but the current of hydrogen was maintained. At room temperature the number of heart-beats, which at the begin- ning of the experiment had been 117, rose to 87 (in twelve minutes), to descend again to 36 in the course of the next hour. Forty minutes later the minimum of 21 was attained, at which rate the heart continued to beat for seven hours. Toward the last a slight increase occurred 1 This experiment shows that the oxygen diffuses comparatively rapidly into the egg. [1903] Digitized by Microsoft® PHYSIOLOGICAL EFFECTS oF Lack or Oxyaren 4138 in the number of beats. I had expected that the number of heart-beats would be only the minimal one after removing the gas-chamber from the ice. Possibly all the oxygen had not been driven out. I therefore repeated the same experi- ment, but allowed the gas-chamber to remain for three hours on the ice. This time I expected that at room temperature the number of heart-beats would only reach the minimum which corresponded to the temperature. But this time also the number of heart-beats rose in six minutes to 66, after which the rate decreased steadily. One hour later the heart beat 42 times, and after thirty-five minutes the minimum of 24 was reached. I do not doubt that after passing a vigor- ous current of hydrogen through the gas-chamber for three hours all the oxygen is exhausted from the egg. If this assumption is correct, these experiments can be made to har- monize theoretically with the results obtained earlier only by assuming that the processes of hydrolysis do not occur with uniform intensity, but that they occur much more rap- idly at first when the oxygen is first withdrawn (or perhaps also under the ordinary conditions of oxygen supply) than in the continued lack of oxygen.’ It is, moreover, to be noted that the data necessary for calculating the work of the heart are lacking in these ex- periments. Only by assuming that these data are the same in the presence of oxygen as in its absence can conclusions be drawn as to the behavior of the two sources of energy. If we make this assumption, we come to the conclusion that of all the energy which is used up by the Fundulus embryo in normal heart-activity, that much at least which cor- responds to the minimal number of heart-beats in the lack of oxygen is dependent upon processes of hydrolysis. This number is about one-sixth or one-fourth of the total number of heart-beats which occur under normal conditions of 1 Perhaps in this case the effects of poisonous substances are to be considered, [1903] Digitized by Microsoft® 414 STUDIES IN: GENERAL PHYSIOLOGY oxygen supply and at the same temperature. When, how- ever, we consider the results of the experiments carried on in the cold, we come to the conclusion that in the presence of oxygen the proportion of energy obtained through processes of splitting may be much greater than this; it may then amount to 50 or 70 per cent. of the work done by the heart. The behavior of the heart of a Fundulus embryo in car- bon dioxide is of interest in so far as it shows that carbon dioxide is just as poisonous in this case as upon the heart of the Ctenolabrus. While the Fundulus heart continues to beat for twelve hours, and even longer, when the oxygen is driven out by hydrogen, the ventricle ceases to beat as early as twelve minutes after passing carbon dioxide through the gas-chamber. Only the auricle continues to beat, and the circulation soon comes to a stop. The contractions become weaker and less numerous. In one experiment the heart beat 96 times per minute at the beginning of the experiment, 54 times after eight minutes, 45 times after ten minutes, and 42 times after twenty minutes. The heart then ceased to beat entirely for long periods of time, and thirty-two minutes after turning on the carbon dioxide the heart stood still. In other experiments the heart did not cease to beat until after one and one-half hours. When the heart is exposed to the poisonous effect of CO, and ceases to beat even after one hour, the heart begins to beat again when the carbon dioxide is replaced by air. The resuscitation of the heart is as follows: The auricle recovers more rapidly than the ventricle, and the latter at first beats a less number of times than the former. In one ex- periment a heart which had come to a standstill was exposed to air at 104. m. At 10:06 the auricle beat 24 times per minute, while the ventricle was still quiet. The ventricle did not begin to contract until the next minute, and the number of auricular contractions was 33 a minute at this Digitized by Microsoft® PuHYSIOLOGIOAL Errerots oF Lack ofr OxyGcen 415° time. At 10:28 the auricle beat 72 times, while the ven- tricle beat 42 times per minute. The ventricle often con- tracted only once to every two, or even at times three, auric- ular contractions. At 10:35, however, the rate of the ventricular and the auricular contractions was the same, namely, 84 per minute, and from then on they continued the same. These phenomena, which are characteristic of all experiments with carbon dioxide, were never observed in replacing the air by pure hydrogen. In another series of experiments I permitted CO, and hydrogen to pass alternately through the gas-chamber. In one case I turned on the hydrogen at 8:30 a.m. At 10:30 the heart beat 24 times per minute; at 10:31 the hydrogen current was interrupted and the CO, current was turned on. (Through a simple T-tube connection and a pair of pinch cocks it was possible to pass either the hydrogen or the CO, through the gas-chamber at will, without admitting air.) In a few minutes the ventricle ceased to beat and the circula- tion stopped. After an hour the current of carbon dioxide was interrupted, and hydrogen was again passed through the chamber. After forty minutes the ventricle again be- gan to beat; the number of its beats was 24, and remained so until death. By replacing the CO, by hydrogen it is therefore possible to do away with the poisonous action of the former. This experiment demonstrates very nicely a fact which is perhaps doubted by no one: that carbon dioxide and lack of oxygen have entirely different effects, which in ordinary cases of asphyxia are added together.’ In this way it is possible by passing through the chamber a current of pure hydrogen gas to bring to life again a ventricle which has been asphyxiated in carbon dioxide. 1It also demonstrates very nicely the possibility that other non-volatile poisonous substances may be formed, by lack of oxygen, which are destroyed again when oxygen is again admitted. The phenomena of fatigue may belong to this category. [1903] Digitized by Microsoft® 416 STUDIES IN GENERAL PHYSIOLOGY Finally, it was of interest to compare the resuscitating effect of air with the resuscitating effect of hydrogen. Fun- dulus embryos were introduced into two gas-chambers. At the beginning of the experiment the heart under observation in one of the chambers beat 90 times a minute; that in the other, 96 times a minute. Hydrogen was passed through the chambers, and after an hour and fifty minutes the frequency of the heart-beats had fallen in both cases to 18 per minute. In place of the hydrogen, carbon dioxide was then passed through the chambers. In fifteen minutes the ventricles stopped beating, and the pulsations of the auricles became much weaker. After 45 minutes one of the hearts was apparently dead, while the auricle of the other still beat 18 times a minute, though the beats were scarcely perceptible. One of the gas-chambers was then opened and the embryo exposed to the air, while in the second cham- ber the CO, was replaced by hydrogen. After fifteen minutes the heart which had been apparently dead and which was exposed to the hydrogen beat 24 times a minute, but only the auricles contracted. Both the auricle and the ventricle of the heart which was exposed to the air beat 60 times. Two hours later the heart beat 30 times per minute in the hydrogen, but the contractions were still limited to the auricle, while the heart exposed to the air beat 72 times. When a little later I exposed the em- bryo kept in the hydrogen to air, the ventricle did not recover. The number of auricular contractions did rise within fifteen minutes from 18 to 54, but shortly there- after the entire heart ceased to beat. In resuscitating a heart. poisoned by CO,, oxygen is therefore more effect- ive than the simple removal of the CO, by hydrogen. We are not able to explain why the ventricle ceases to beat when exposed to carbon dioxide sooner than the auricle. We meet with an entirely different relation between car- Digitized by Microsoft® PuHyYsrtoLoGicaL Errects or Lack of Oxygen 417 diac activity and oxygen in the larve of a fresh-water mussel (Cyclas). In this animal the frequency of the heart- beat steadily decreases from 50 heart-beats to 0 in the course of one and one-half hours in an atmosphere of hydrogen (at 24° C.). In this case, therefore, we have neither a sudden standstill of the heart without an appreciable decrease in the frequency, as in Ctenolabrus, nor a long-continued steady beat of low frequency, as in Fundulus, but a decrease in cardiac activity which runs parallel with the removal of oxygen, as if processes of oxidation are the sole source of energy for the activity of the heart. XI. ON THE TRANSFORMATION OF NEGATIVELY HELIOTROPIC ANIMALS INTO POSITIVELY HELIOTROPIC THROUGH LACK OF OXYGEN A series of papers have proved that it is possible to change the sign of heliotropism in certain animals at will through external conditions." It is an easy matter, for example, to render negatively heliotropic Copepods posi- tively heliotropic by cooling, and to keep them permanently positively heliotropic at a low temperature; while it is also possible to render positively heliotropic Copepods negatively heliotropic by an increase in temperature. The same experiments can be made on larve of Polygordius. In order to determine the cause of this change in the sign of heliotropism, and also the conditions upon which the latter depends, I tried to see whether other conditions could bring about similar changes. Groom and I had previously found that the positively heliotropic Nauplii of Balanus perforatus rapidly became negatively heliotropic when exposed to strong light. I also found that the same effect can be produced upon Copepods and Polygordius larve by properly diluting the 1GRoom uND LOEB, Biologisches Centralblatt, Vol. X; Lorn, Vol. I, pp. 265 ff. Digitized by Microsoft® 418 STUDIES IN GENERAL PHYSIOLOGY sea-water as by increasing the temperature, while a proper increase in the concentration of the sea-water brings about the same effect as cooling. The majority of Copepods were, immediately after being caught, positively heliotropic. It seemed as if the majority of the negatively heliotropic Copepods belonged to one and the same species. When the Copepods were allowed to remain for a long time in a vessel containing sea-water, the number of negatively heliotropic animals decreased, becom- ing positively heliotropic with time, while the reverse change occurred only rarely. The experiments on the effect of lack of oxygen were made under a small bell-jar, the contents of which were separated from the air on the outside by mer- cury. Two tubes extended into the bell-jar, one of which conducted the hydrogen into the bell, while the other con- ducted it away from the bell. Two vessels were placed under the bell-jar, of which the one contained freshly selected positively heliotropic Copepods, while the other contained negatively heliotropic Copepods. While the positively heliotropic animals remained positively heliotropic during the course of the experiment, the negatively helio- tropic Copepods within fifteen to twenty minutes after the hydrogen was turned on began, in part, to leave the room side of the vessel and to distribute themselves irregularly throughout the vessel, in part to collect at the window side of the vessel. The number of animals collected near the window steadily increased, while the number of Copepods at the room side of the vessel steadily decreased. In about thirty to forty-five minutes after the current of hydrogen had been turned on all the Copepods lay quietly on the bottom of the vessel. The Copepods which had from the beginning been positively heliotropic died at the side of the vessel nearest the source of light. Most (if not all) of the Copepods which had at first been negatively heliotropic Digitized by Microsoft® PHYSIOLOGICAL EFFects oF LAok oF Oxycen 419 were also found at the window side of the vessel. A second small collection occurred in the middle of the vessel, while the room side of the vessel was entirely vacated. The ani- mals usually did not become positively heliotropic until shortly before they became motionless. This explains why the conversion of the negatively heliotropic into the posi- tively heliotropic animals through lack of oxygen cannot be obtained with the precision and elegance with which the change can be obtained by cooling. In the latter case the animals retain their full power of movement; in the former the transformation does not occur until the animals have suffered from lack of oxygen. But even then the phenome- non is so striking that it might be used as a demonstration experiment. JI have repeated the experiment eight times with the same result. At first it seemed to me as if the negatively heliotropic Copepods died more rapidly in the absence of oxygen than those which were positively helio- tropic from the beginning. This finding, however, was not borne out in every case. When the experiment was interrupted early, at a time when the animals first began to become positively heliotropic, and air was then admitted, the Copepods which had become positively heliotropic again became negatively heliotropic. The remarkable effects, which we have described here, of lack of oxygen on the sense of heliotropism, are, of course, not confined to Copepods. I made similar experiments upon the negatively heliotropic marine Isopods, the majority of which also become positively heliotropic in less than two hours when oxygen is withdrawn. These experiments will be continued. We see, therefore, that lack of oxygen has the same effect upon the sense of heliotropism as cooling or increas- ing the concentration of the sea-water. Araki has shown that by cooling the chemical effects of lack of oxygen can Digitized by Microsoft® 420 STUDIES IN GENERAL PHYSIOLOGY be brought about, and it is therefore possible that the posi- tive heliotropism in both cases is determined by the same chemical conditions. It must be left for further experiment to decide this point. XII. ON CHANGES IN PIGMENT CELLS IN LACK OF OXYGEN It is a definitely established fact that the pigment cells in the skin of the frog become lighter after death. This lightening is brought about, as Biedermann has found,’ by the fact that the coloring matter collects into small clumps. A piece of the skin which has been deprived of its circu- lation shows the same changes. In the transparent portions of the skin which can be studied microscopically —such, for example, as the web of the amputated foot of Rana temporaria—it can easily be seen how the much- branched pigment cells which follow the course of the capillaries gradually change their form, in that the coloring matter moves toward the center of the cell until finally all the pigment is col- lected into clumps (p. 175). Increase in the carbon dioxide cannot be the cause of this change in the pigment cells, for Biedermann found that the skin does not become lighter when the frog is poisoned with CO,. Biedermann believes that the cause is probably to be found in the decrease in the amount of oxygen. The surface of the yolk-sac of the Fundulus embryo is studded with a large number of black and reddish-yellow pigment cells, which are at first distributed irregularly, but which later, as I have shown,” are compelled to creep upon the blood-vessels and surround them. With this the first physiological cause was furnished for the marking of an animal. Since then other authors have also found that the course of the embryonic blood-vessels determines the mark- ing of the embryo. 1 Pfliigers Archiv, Vol. LI, 2 Journal of Morphology, 1893. Digitized by Microsoft® PuHYSIOLOGIOAL Erreots or Lack oF OxyGen 421 The black and red pigment cells can be distinguished from each other, not only by their color, but also by their form. The latter send out a large number of thin pseudo- podia-like processes which are never found in the black pig- ment cells. In the experiments on the effect of lack of oxygen on the cardiac activity of the Fundulus embryo, it was noticed that the originally dark yolk-sac gradually be- came lighter in color when exposed to hydrogen for a long time. The pigment cells can be observed very carefully with the microscope, and I expected to observe the same phenomena that Biedermann observed in frogs. This was, however, not the case. It was found in the course of a series of experiments that the dark pigment granules and the black cells gradually disappear the longer the current of hydrogen is kept up, and that the collection of the pigment in the center of the cell does not occur. The changes in the red pigment cells in lack of oxygen are of a somewhat different nature. The lightening of the color often occurs in this case also. Besides this, however, the cells become smaller. The tips of the cell-processes break off, remaining visible at first as tiny droplets, which disappear later. As this process continues, the pigment cells gradually become smaller. These changes remind one of the fact that certain dyes become colorless when reduced. In our experiments it might also be possible that the discoloration of the black pigment is a result of a reduction, which does not occur in the presence of atmospheric oxygen. XIII. CONCLUDING REMARKS It seems to me that the most important result of the foregoing experiments consists in the proof which has been brought forward that in certain cases at first molecular, and later morphological, changes are brought about in cells Digitized by Microsoft® 422 STUDIES IN GENERAL PHYSIOLOGY through lack of oxygen, which in their turn are the cause of the suspension of life-phenomena. This has been proved for the process of cleavage in the Ctenolabrus egg. The cleavage-cells of Ctenolabrus are dissolved again and fuse together when oxygen is removed. These changes are not, however, an evidence of death, for as soon as such a fused blastoderm is again exposed to air it begins to divide anew. On the other hand, these molecular changes are sufficient to hinder cleavage. The cleavage-cells of the Arbacia egg seem to suffer similarly in the lack of oxygen, although the changes are much less marked. We find that here also cleavage is impossible without oxygen. Yet lack of oxygen does not bring about the same sort of molecular changes in the Fundulus egg as in the Ctenolabrus egg, and corre- sponding with this difference cleavage may also go on with- out oxygen for many hours in Fundulus. It is also possible that such molecular changes as are brought about by the lack of oxygen in the cell are also the cause of the cessation of other life-phenomena; for example, the beat-of the heart (and the activity of the respiratory center). We thus find that in Ctenolabrus, where the first cleavage-cells suffer such profound structural changes through lack of oxygen, the heart of the embryo comes to a standstill very rapidly and suddenly through lack of oxygen before a marked decrease has taken place in the frequency of the heart-beats; while the heart of the Fundulus, whose cells suffer no such structural changes, continues to beat for many hours without oxygen. Since the chemical energy set free in the cells must first be converted into molecular energy in order to bring about the physiological function, it is clear, a priori, that not only a decrease in the supply of the chemical energy, but any structural change which ren- ders impossible the conversion of chemical energy into the molecular energy necessary for the activity of the tissue, Digitized by Microsoft® \ PHYSIOLOGIOAL Erreots oF Lack oF OxyGEn 423 must also lead to a standstill in the particular life-phenome- non under observation. We find that both possibilities are, indeed, encountered. Further investigation may possibly be able to show that the sudden cessation of life-phenomena through lack of oxygen may generally be attributable to structural changes. Were this the case, it might point the way, perhaps, to a clearer understanding of the action of many poisons. 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