The Dynamics of Living Matter

The galvanotropic reactions were first discovered in vertebrates. Purkinje noticed that if a galvanic current is sent through the brain of a human being, sensations of motion and dizziness are produced. Brenner recognized the polar character of this effect, and found that if a current of sufficient intensity is sent laterally through the head, the person falls toward the anode side upon making the current, toward the cathode side, upon breaking the current. Mach noticed that if a current is sent sidewise through fishes, the animals have a tendency to roll toward the anode side.f

The introduction of the term "galvanotropism" into physiology dates from J. Mliller-Hettlingen, who found in Hermann's laboratory that if the seedlings of Vicia faba are exposed to a constant current, the tips of the roots bend toward the cathode. { Hermann soon afterward made similar experiments on the larvas of frogs. He found that these animals, when put into a trough through which a current goes, are turned into the direction of the current curves, putting their heads toward the anode. § I must, however, admit that I never succeeded in repeating this experiment on tadpoles.

Blasius and Schweizer|| found that a large number of animals, when put into a trough with water through which a constant current goes, have a tendency to go to the anode. They assume that the current acting upon the central nervous system causes sensations of pain when it goes in the ascending direction through the animal; while it calms the animal when it goes in the opposite direction (from head to tail). * Quoted from Loeb, Comparative Physiolog\> of the Brain and Comparative Psychology, New York, 1900.

The animals according to these authors choose the position which oc- casions the least pain; namely, with their heads toward the anode. This assumption is contradicted by the above-mentioned experiences on the effects of the galvanic current on the brain of human beings, which show that the tendency to fall toward one side is not produced, and not even accompanied, by any sensation of pain. Moreover, the above-mentioned observation on the effects of the current on the ten- tacles of Polyorchis, the reactions of Paramacium, and the observations on Pal&monetes, show that these reactions find their adequate explana- tion in the direct effects of the current upon the organs or nervous mechanism of locomotion ; that there is no room left for the smuggling in of hypothetical pain sensations between the current and its effect upon the mechanism of locomotion. I have repeatedly pointed out that it is superfluous, and often in direct contradiction to the facts, to assume the existence of human sensations in lower animals, and to put these hypothetical sensations as a necessary link between the ex- ternal stimulus and its motor effect. It is easy to see what led Blasius and Schweizer to their assumption. If we send a current through a trough in which are found specimens of a Salamander, Amblystoma, the attitude of the animal changes considerably, according to the direc- tion in which the current goes. When the current goes from tail to head, the animal assumes an opisthotonic position, with its mouth open. It is evidently this condition, together with a certain restless- ness, which caused Blasius and Schweizer to assume that the ascend- ing current excites the animal painfully.

If we, however, look at the condition of the animal when the current goes from head to tail, we see that in this case the animal is also in a forced position ; namely, with its head downward and its back convex. The right expression of the facts is, it seems to me, that the descending as well as the ascending current change the tension of certain muscles; but while the latter causes the contraction of the extensors of the vertebral column and of correlated muscles, the former causes the contraction of the flexors. The assumption of the pain sensation as the necessary link in the one case and not in the other is quite arbitrary and unnecessary. It is probable that in animals possessing a central nervous system, the galvanotropic reactions are brought about chiefly by the action of the current upon the central nervous system. Since the galvanic current influences not only the superficial layers of an organism like the light, but penetrates through the whole body, the cases of ideal galvanotropic orientation are not so common as those of heliotropic orientation.

It is well known that even in the dark the tips of the main roots of many plants show a tendency to grow vertically downward, while the tips of the main stem show the opposite tendency. If such plants are put into a position other than vertical, the tip bends until the vertical lines strike symmetrical points at the same angle. In such cases we call the roots positively, the stems negatively, geotropic. Knight has shown by putting plants on a rotating disk that these effects are due to gravitation. In a centrifugal machine the tips of the root grow toward the periphery, the stems toward the center of the disk. In grasses the curvature occurs in the nodes, while in other forms it occurs in the growing region near the tip of the root or the stem.

While chemistry furnishes sufficient data for the assumption of photochemical effects in organisms, we do not know of any direct effect of gravitation upon chemical reactions. Eight years ago I pointed out that such an effect might occur in this way ; namely, that in the cells, or in certain cells, of geotropic organs, nonmiscible substances (e.g. solids and liquids) might exist, and that by the change in the position of the organ a change in the relative position of these phases might be brought about.* This change in position might be connected with an acceleration of the reactions on the one side, and the reverse effect on the opposite side of the organ. I was led to such an assumption by the observations made on the resting muscle in a stretched and normal condition. If the excised muscle of a frog is stretched passively by a weight, it produces more lactic acid than in the unstretched condition.f I am inclined to attribute this effect of the stretching merely to a change in the form of the muscle. It might be possible that the stretching increases the surface of certain (the anisotropic?) elements in the muscle, whereby the area of contact (with the isotropic substance?) and therefore the reaction velocity might be increased. Something similar might happen in geotropic organs, when they are put into a hori- zontal position. Suppose that in the normal (upright) condition of the stem certain solid or viscous substances (e.g. nuclei) of a higher specific gravity than the other constituents of the cell lie at the base of

each cell of the tip of the stem (Fig. 32). If, however, the stem is put into a horizontal position, these heavier particles will go to the peripheral side of the cells on the lower half of the tip and to the central side of the cells on the upper half of the tip (Fig. 33). This difference in the position of these solid particles may determine differences in the reaction velocity of the chemical processes in both groups of cells. In the cells on the upper side the heavier elements are in more direct contact with the substances diffusing into the cells from the

pith, while in the cells on the under side the reverse is true. These assumptions are purely speculative, serving only as an illustration of the statement that a change in the position of an organ might influence the reaction velocity on the upper and lower sides of the organ differ- ently. In consequence of such an influence, a curvature like that in heliotropic reactions might be produced. Czapek has found chemical differences between the tips of roots which were put into a horizontal position and those left in their normal vertical position. In the extreme tips of the positively geotropic roots of Lupinus albus the amount of homogentisinic acid increased about 15 per cent in about half an hour when put into a horizontal position. At the same time, a retardation in the blueing of tincture of guaiacum was noticeable, which he considered the effect of the formation of an antioxidase.* It is, however, questionable, whether these chemical changes are responsible for the geotropic curvature. In order to prove this it would be necessary to show a chemical difference in the lower and upper sides of a root or stem which had been put or brought into a horizontal condition. Czapek, however, states that he could not find any difference between the upper and lower sides.

Animals also show geotropic phenomena, f Antennularia, a Hy- droid, behaves toward gravitation like a geotropic plant, and it would be possible to demonstrate the principle of geotropic curvatures in this animal. When the stem of Antennularia antennina, which normally grows vertically upward, is put into an oblique position in the aqua- rium, the tip bends until it is again in a vertical position, and then con- tinues to grow in this direction vertically upward. The roots are not quite so straight as the main stem, and although thev have a tendency

to grow down vertically, the geotropic reaction is not so precise as in the stem. The latter is negatively, the former, positively, geotropic. In free-moving animals, geotropism is not so rare; Cucumaria cucumis, a Holothurian, possesses five rows of feet with which it can creep on ver- tical surfaces. If the animal be put on a vertical glass plate, it will creep vertically upward. When the plate is turned very slowly around a horizontal axis, the animal remains quiet during the rotation, but as soon as the plate is fixed, the Cucumaria again creeps vertically up- ward. This occurs also in the dark room. We will call such animals that are compelled to creep vertically upward, negatively geotropic.

Many marine animals and many insects show this reaction. If free-swimming aquatic animals which show a tendency to gather at the top or the bottom of a vessel are used for experimentation, the ex- perimenter must be careful not to mistake the passive sinking or rising of such forms for geotropic reaction. Ostwald has called attention to the fact that with increasing temperature the internal friction of the water diminishes rapidly, which necessitates that organisms which float at the surface at a lower temperature must either sink down at a higher temperature, or are unable to work upward, on account of the diminished internal friction of the water. In addition, the resistance due to the shape of the animal plays a role in these phenomena.*

The attempt which many animals make to keep the axes of their eyes as nearly as possible in their normal position in space when the body is put into an abnormal position, is a common reaction which seems to be determined by gravitation. In Crustaceans the eyestalks form a small angle with the horizontal plane when the animal is in its normal position. When turned on one side, however, so that the right side is directly downward, the eyes no longer keep their symmetrical position in regard to the plane of symmetry of the animal, but the right eye is raised, the left lowered. f It looks as if the eyes had a tendency to keep their former normal position in space ; just as the root or stem of a geotropic plant tries to keep its orientation toward the center of the earth. This reaction of the eyes also exists in vertebrates, and can be nicely demonstrated in fishes, lizards, birds, or rabbits. In frogs the eyes do not show the so-called compensatory motions, but the head as a whole tries to keep its normal orientation toward the horizon, when the body of the animal is put into an abnormal posi- tioQ. These reactions exist universally, but in such forms as possess a powerful associative memory the reaction is liable to be interfered with.

Knight's experiment can be made in these forms. When they are put on a rotating disk and turned in a horizontal plane, the eyes are displaced during the rotation in the plane of the rotation, but in the opposite direction. In this case again a source of error has to be guarded against; namely, the influence of the retinal image. The tendency to keep the eyes and the head in the normal position to the horizontal when the body is turned, may be determined by the influence of the visual impres- sions. The apparent motion of the objects on the retina when the animal turns may cause a motion of the eye and head in the opposite direction. This suspicion is the more justified as in some insects these compensatory motions cease when the eyes are blackened.* Lyon has, however, shown that in sharks and flounders these compen- satory motions are not diminished when the optic nerves are cut. In these forms at least we seem to deal with really geotropic reactions.

The next question must be, In which organs are these geotropic reactions produced? The answer might be simple enough were this field not in a hopeless state of confusion through the hypothesis con- cerning the functions of the semicircular canals. Flourens had stated that the sectioning of one of the semicircular canals causes the eyes and head of the animal to move in the direction of the plane of the canal. Goltz showed that destruction of the inner ear leads to disturbances "of the maintenance of the equilibrium of the animal." This term, "equilibrium," is not clear unless it be supplemented by the statement "with reference to the horizon or to the center of the earth." Goltz advanced the now famous hypothesis that semicircular canals are an. organ where the equilibrium of the head, and indirectly of the whole animal, is regulated. If the head is bent, according to Goltz, the flow of the lymph in the canals causes a stimulation of the nerve endings in the ampullae of these canals, and this calls forth a reflex motion, by which the head is put back into its normal position. Mach showed that physical reasons prevented a flow of lymph such as Goltz's hypothe- sis demanded; but that the pressure of the lymph against the nerve endings in the ampullae, caused by changes in the position of the head, might suffice to bring about the effects which Goltz's hypothesis de- manded. He showed, moreover, that this hypothesis also demanded that any stimulation of one of three ampullae only called forth a motion of the eyes and head in the direction of the canal, and no other. While this hypothesis at first met with general opposition, it was later accepted, and it has, among others, received some support from my own laboratory. Nevertheless, the hypothesis is wrong. Lyon has shown

conclusively in my laboratory that the stimulation of the horizontal canals in sharks and flounders calls forth motions of the eyes in the plane of the canal, as the hypothesis demands, but that the two other canals, or their ampulla;, are either nonsensitive to stimula- tion, or give no motion of the eyes or the head in the plane of the stimulated canal. I will confess that I did not at first credit Lyon's statements, but I have convinced myself that he is unquestionably right. It seems that all the authors who had stated that stimulation of one of the semicircular canals caused motions of the eyes or head in the plane of the canal, based their statements only on the effects of the stimulation of one of the three canals; namely, the horizontal. The negative or questionable results they obtained in the case of the two vertical canals they did not dare to accept in the face of the strik- ingly clear results the horizontal canal yielded.* Suggestion does not play a role in ordinary matters only, but occasionally also in science.

It is, however, possible that the compensatory motions and reactions are after all produced in the inner ear, although the semicircular canals have little or nothing to do with it. This follows from the fact that when the auditory nerve in a shark is cut, all the compensatory motions cease.f It may be that the otoliths in the inner ear are responsible for this effect. Mach J was the first to point out this possibility, and De- lage § made a number of experiments which seemed to speak in favor of this view. The most striking experiment was made by Kreidl on Pal&mon. The otolith organs of this Crustacean are found in the basal part of the small antennae. Palcemon loses its otoliths during the process of moulting, and after moulting it repairs the loss by putting small granules of sand into the ear. Kreidl kept such Crustaceans in vessels which were free from sand, but which contained instead very finely powdered iron. After moulting the animals put this iron powder into their ears. It was possible to test in such animals the otolith- hypothesis of the geotropic reaction. The otolith rests on cells in which the sense nerves end. They are therefore supposed to press upon the nerve endings. When the animal is laid on one side, the otolith, instead of pressing, will pull at the cell, and this causes a change in the nerve endings which results in a righting motion (compensatory motion) of the eyes, or, if possible, of the whole body. If this view were correct, it should be expected that a magnet could produce effects similar to

* It is much easier to ascertain motions of the eyeball from right to left or vice versa than up and down. This is due to the fact that we estimate the motion from the displace- ment of the sclerotic. In the upward and downward motions, however, the sclerotic is, as a rule, not visible. J Mach, Grundlinien der Lehre von den Bewegtingsempfindungen, Leipzig, 1875. gravitation, when the otoliths were of iron. If, e.g., the magnet were approached from the right side of the animal, the iron otolith would be pulled toward the right, and this should result in a reflex turning of the animal upon its left side. Kreidl found indeed that this occurred.* Kreidl's experiments were repeated and confirmed by Prentiss.

Delage, Kreidl, and Lyon all agree that the "maintenance of equi- librium " or more correctly speaking the geotropic reactions of the animal do not entirely disappear when the small antennae are cut off. This proves that another organ contributes to these reactions, namely, the eyes. Removal of the eyes and the antennae does away with the compensatory motions. While it is thus probable that the otoliths have something to do with the reactions of the animal, it does not seem as if this were gen- erally the case. The flounder possesses a single large otolith in each ear, which can easily be extracted without injury to the ear. Lyon found that if the otoliths were removed, the geotropic reactions and "maintenance of equilibrium" were not disturbed.

This field requires further investigation, and I should not be sur- prised if it were found that the really geotropic reactions of animals were determined in certain cells of the inner ear, or in certain cells of the brain, while otoliths may or may not act in an accessory way. It would, however, be a mere anthropomorphism to assume otolith organs inside the cells (as some botanists now begin to do for plants). Inside of the cells of geotropically sensitive organs gravitation may probably act through an influence upon the reaction velocity of certain chemical processes, as set forth at the beginning of this chapter.

Theoretically we may assume that if substances diffuse in air or in water, the particles move in a straight line away from the center of diffusion. If they strike an organism whose surface is affected by the diffusing substances on one side only, the contractile proto- plasm, or the muscles, turning the tip or the head or the whole organ- ism toward that side, are thrown into a different state of contraction from their antagonists. The consequence is a turning or bending of the tip or the head until symmetrical points of the chemically sen- sitive surface of the body are struck by the lines of diffusion (or the diffusing particles) at the same angle. As soon as this occurs the con- tractile elements on both sides of the organ, or organism, are in an equal

state of contraction, and the animal will bend or move in the direction of the lines of diffusion. There is practically, however, this difficulty; namely, that the lines of diffusion are generally disturbed by currents due to changes and variations in temperature, and instead of the straight lines of force we have in this case often irregular and changing ones. This makes it a priori hopeless to expect that in the case of chemo- tropism the organisms move in as straight a line as in that of heliotro- pism, geotropism, and certain cases of galvanotropism. In the majority of cases we are also dealing with a response to sudden changes in the nature and concentration of the substances contained in the medium.

Engelmann was probably the first to call attention to this type of phenomena. He found that certain bacteria and Infusorians gather around a source of oxygen.* In this case it was evidently a response to changes in the concentration of the oxygen, the organisms coming to rest where the tension of oxygen was a relative maximum. Pfeffer first compared these phenomena with those of other tropisms in his classical paper "Oriented Locomotor Motions produced by Chemical Stimuli." f He showed that the zoospermiae of ferns, mosses, and other plants move toward points from which certain substances diffuse into the water in which these organisms are. Pfeffer found that such effects are produced upon spermatozoa of ferns by malic acid and its salts, and upon those of mosses by cane-sugar solutions. The biological importance of this observation lies in the fact that malic acid is com- paratively common in plants, and the presence of this acid in the arche- gonia of the ferns possibly contributes toward bringing the sperm to the egg. From the normal archegonium no malic acid diffuses, but those ready to be impregnated let part of their contents diffuse. The appearance of Pfeffer's paper aroused in many the hope that it might be shown that the animal egg, too, attracted the spermatozoa in some such chemotropic or chemotactic way; but all the experiments thus far made in this direction by J. Dewitz, Buller, and others - - 1 have made quite a few experiments myself on this subject - - have without exception shown that such is not the case in the eggs thus far tried. There does not seem to exist an attraction of the spermatozoa on the part of the egg, but the meeting of spermatozoa and the egg is left to chance, except that automatic tropismlike mechanisms exist, whereby the ripe males gather near the ripe females and the sperm is shed in the neighborhood of the egg.

Pfeffer's method consisted in introducing a solution of the chemical substance to be tested, e.g. malic acid, into a capillary tube which was sealed at one end, and then putting the tube with the open end into the water which contained the spermatozoa. "When the liquid in the tube contains only o.oi per cent malic acid, the spermatozoa (of ferns) very soon move toward the opening of the capillary tube. At the same time many spermatozoa move into the capillary tube and within from five to ten minutes many hundreds of spermatozoa may accumulate in the tube. The malic acid acts as well in the form of a free acid as in the form of salts, and that it is a specific stimulant may be seen from the fact that in the same time prob- ably not a single spermatozoon enters a capillary tube containing pure water or a solution of other substances." Massart and Bordet used this method of Pfeffer's on leucocytes.* It had been known for a long time that in inflamed tissues the number of leucocytes increases, and it was generally admitted that at least part of the supernumerous leu- cocytes migrate there from the capillaries. In order to answer the question as to what causes this migration, Massart and Bordet put capillary tubes containing cultures of bacteria, especially staphylo- coccus pyog. aureus into the abdominal cavity of the frog. After twenty- four hours the authors found leucocytes in large numbers in the tube. If a sterile culture medium was introduced, no leucocytes migrated into it. This seems to indicate that substances produced by the bacteria determine the direction in which the leucocytes move.

To give a more distinct picture of these phenomena I may mention a few of the observations made by Garrey f on this subject. A small square trough contained the organisms - - in this case Chilomonas, an Infusorian. At one side a small capillary tube was inserted, into which the solution of the substance was put whose efficiency was to be tested. At the beginning of the experiment the organisms were equally distrib- uted all over the square space. When very dilute HC1 was put into the capillary tube, a clear circular area entirely free of organisms was soon formed around the opening of the tube. The organisms recede from the HG1, diffusing into the trough, and thus indicate very nicely the rapidity and extent of diffusion. This clear area increases until the HC1 has diffused to the end of the square space, when the organ- isms are again distributed equally. In this case we are probably not dealing with a real tropism, but with a reaction to sudden changes in the intensity of the stimulus. When the organisms go from neutral water to sufficiently acidulated water, they are repelled. According to Jennings, they swim first backward and then toward one side, a

reaction which is quite common among Infusorians, and to which Jennings has given a special name, motor reflex or motor reaction.* Garrey found that the phenomenon described above can be produced by many inorganic acids, provided their concentration is — • n or above. Alkalis bring about the same effect at a somewhat higher concentration; namely, - n. Salt solutions require a still higher concentration, e.g, NaCl, arid LiCl require a minimum concentration

at a lower concentration ; namely,' to - -. ZnSO., ZnCl2, CuSCX, The immense biological role of these reactions is known to every one who has worked with insects. The finding of food, the depositing of eggs, and the meeting of the two sexes for the process of pairing are determined to a large extent by diffusing substances. I may relate the following observation f which certainly has been made often enough before. A female butterfly was put into a small, closed wooden box which was suspended from the middle of the ceiling of a room whose windows were open. At first no other butterfly of the species to which the female belonged was visible, but during the next half hour three male butterflies of the same kind approached the house, stopped at the window, then flew into the room, and settled on the wooden box through the openings of which they tried to enter. This effect could have been produced only by an emanation from the female butterfly. As an example of how emanations direct the motions of females that are ready to deposit their eggs, the fact may be cited that certain volatile sub- stances emanating from meat ''attract" the female fly. If fat and meat of the same animal are put side by side on the window sill, the female fly will light on the meat, where she deposits h'er egg, but not on the fat. This tropismlike reaction guarantees the perpetuation of the race, inasmuch as the larvae feed and develop on meat, but not on fat.

Certain animals are compelled to put their bodies as much as pos- sible into contact with solid bodies, while other organisms show the * Jennings, Am.Jottr. Physiology, Vol. 2, p. 374, 1899 ; and numerous subsequent papers by the same author. t Loeb, Animal Heliotropism and its Identity with the Heliotropism of Plants, Wiirz- burg, 1889. reverse behavior. The former organisms I designated as positively, the latter as negatively, stereotropic. The first discovery in this direction was made by J. Dewitz,* who found that the spermatozoa of the cockroach (Periplaneta orientalis) are "attracted by surfaces." "If small pieces of glass or some other object are placed between a slide and a cover-glass so that there is a space between cover glass and slide, and if this space be filled with a NaCl solution containing the spermatozoa, the latter gather only at the cover glass and the slide. In the rest of the liquid no spermatozoa are found. If a glass bead be put into such a liquid containing sper- matozoa, the latter in no case leave the surface of the bead, although they are constantly in motion." Dewitz recognized that this reaction was of the greatest importance for the entrance of the spermatozoa into the egg of the cockroach. This egg possesses a micropyle, and only here can the spermatozoa enter the egg. When the egg is laid it passes the duct of the seminal receptacle, where the female carries the sperm it receives in the act of pairing. The egg then comes in contact with the sperm, some of which is possibly pressed out of the receptacle. rerlexly by the passing of the egg. When once on the surface of the egg, the spermatozoa can no more leave it, but must move on its sur- face incessantly. In this way one spermatozoon finally reaches the micropyle and gets into the egg. The impregnation of the egg is there- fore in this case a function of the stereotropism of the spermatozoa.

in this case lines of force do not exist, there exist stereotropic curva- tures. When the stems of Tubularia are fixed in an aquarium in such a way that the polyp touches the wall of the aquarium (Fig. 34), the polyp begins to bend away from the wall until at right angles with it, and then continues to grow in this direction. The stolon, however, sticks to the glass wall, possibly by the secretion of a sticky substance. It had been known that a number of animals hide in crevices. This phenomenon was generally ascribed to a supposed timidity or photophobia of these animals, which were believed in this way to pro- tect themselves from their enemies. I showed that in this case the ani- mals are forced to bring their bodies as much as possible in contact with solid bodies. Amphipyra is an outspokenly positively heliotropic butterfly which has a tendency to creep into crevices. If a number of these animals are kept in a box and a plate of glass is put on the bottom of the box so that it rests upon pieces of glass just high enough to allow the Amphipyra to creep under the glass plate, all of the butter- flies will be found after a time collected under the plate. This happens as well when they are in the dark as when the plate of glass is exposed to full sunlight. As long as they cannot creep into crevices they run around restlessly, while they become quiet as soon as their bodies come in contact on all sides with other solid bodies.

The crevices thus act like a trap where such animals are gradually caught until metabolic changes (need of food) again make them rest- less, and compel them to move about.* A similar form of irritability exists in ants. When sexually mature ants are kept in boxes containing pieces of folded paper or cloth, all of these animals will be found after some time in the folds, even if the box is absolutely dark. This form of reaction leads to the foundation of a nest, inasmuch as the female, after pairing, creeps into a crevice, where it lays its eggs.