The Dynamics of Living Matter

This form of irritability is also found in worms. If, e.g., earth- worms are kept in a glass vessel with a horizontal bottom and vertical walls, they collect and crawl in the angle between the vertical and horizontal side. Experiments which S. S. Maxwell made on Nereis, a marine Annelid, show how great the force is which keeps such animals in contact with solid bodies. These animals burrow in the sand. If they are kept in a porcelain dish, into which a number of glass tubes have been put which are just large enough to allow a Nereis to enter, it will be found that in about twenty-four hours each tube will contain a Nereis. The animals cannot even be induced to leave the tube if the latter is exposed to direct sunlight, which kills them, although by crawling out they might save their lives. f There are other forms which avoid contact with solid bodies as persistently as the animals thus far mentioned seek it. This form of irritability, negative stereotropism, is found in many swimming forms, e.g. the nauplii of Balanus.

Positive stereotropism is apparently that form of irritability which next to chemotropism is most instrumental in bringing about the union of the two sexes. The holding of the female by the male during copu- lation is evidently in many forms purely a form of stereotropism. In frogs such contact irritability develops during the spawning season, on the ventral side of the chest. At that time the contact of the ventral side of the chest with any solid body causes a reflex closing of the arms of the male frog around the solid body. The embrace becomes lasting, however, only in case the embraced object is a female frog. In this case, obviously, other stimuli contribute toward making the embrace last- ing. What the nature of these stimuli is, is not yet known.* Holmes has called attention to the fact that the embrace of the female Gammarus by the male is a similar case of stereotropism.f These reactions can even be demonstrated in the decapitated frog.

It is obvious that the tropisms furnish the understanding for many purposeful instinctive reactions, and that what is generally called an instinct is often nothing more than a compulsory turning and moving of an organism in a given direction. I have carried out such an analysis of animal instincts in another book, and therefore do not wish to enter upon this subject here. I believe, indeed, that the tropisms and trop- ismlike reactions will one day form the main contents of a scientific psychology of lower forms. The tropisms, however, and tropismlike compulsory reactions also play a role in the mutual arrangements of organs and tissues. The first case of this kind mentioned was the observation that the tigerlike coloration of the yolk sac of the Fundulus embryo is due to a creeping of the chromatophores upon the blood vessels. At first the chromatophores and blood vessels are formed with- out any definite relation to each other, but by and by every chromato- phore creeps on the capillary, enveloping it completely. % I am not able to state whether this is a case of chemotropism caused by the oxygen in the capillary tubes, or a case of stereotropism. Driesch § suggested later that the migration of the mesenchyme cells to those spaces in the gastrula of the sea urchin where the skeleton is to be formed, might be due to a tropism. Herbst H has pointed out the possibility of a wide application of the tropisms in ontogenetic processes.

* Goltz, Beitr'dge zur Lehre von den Nervencentren des Frosches, Berlin, 1869. || Herbst, Ueber die Bedeittung der Reizphvsiologie fur die causale Aitffassung der Ontogenese, Biologischts Centralblatt, Bd. 14 and Bd. 15, 1894 and 1895. The more fertile a principle is, the more we can afford to be conserva- tive in applying it. It is obvious that certain reactions have been called tropisms which have nothing to do with them; possibly Roux's cyto- tropism belongs to this group. Roux has observed motion of the cleav- age cells of the germ of the frog's egg to and from each other; he has called these cases cytotropism. Driesch has pointed out that these are phenomena which are caused purely by capillary forces between the eggs. If this be correct, as it seems to be, and if we are not dealing in this case with a reaction of living matter to an outside stimulus, we are not deal- ing with a tropism ; for by the latter we mean distinctly a class of com- pulsory reactions of the organism to outside stimuli ; but not the passive motions of bodies caused by capillary forces. That these bodies consist of living protoplasm does not influence this discrimination.

Another warning to be careful in applying this principle was shown by recent investigations of E. P. Lyon.* It is a well-known fact that many fishes put their bodies into the direction of a current of water, and try to swim against the current. It was commonly supposed that this orientation was caused by the streaming of the water, possibly its fric- tion against the sides of the body. Lyon has shown that this behavior is an optical reflex caused by the apparent motion of the object while the animal is moved passively by the stream. When he inclosed the fish in glass jars and dragged these jars through the water, the fish inside the jars oriented themselves in the direction opposite to that in which the jar was moved. There is no objection to calling this a tropic re- action, but it is certain that it should no longer be called rheotropism.

Attention should also be called to the fact that while the tropisms form in many cases the mechanism by which the preservation of the individual and the species is brought about, there are many cases of tropism which are of no use to the species ; the whole field of galvano- tropism is an example of this. Galvanotropism is purely a laboratory phenomenon; outside of the laboratory no animal ever comes into a situation which might call forth a galvanotropic reaction; yet galvano- tropism is not uncommon among animals. Among the positively helio- tropic animals, we find forms which are never exposed to the light, e.g. the caterpillar of the willowborer, or Cuma Rathkii, a Crustacean which lives in the mud at Kiel. I pointed out sixteen years ago that these cases speak against the assumption that the tropisms could have been acquired by the way of natural selection,! and Morgan has recently taken the same ground ; J but I do not wish to enter upon a criticism of

the principle of natural selection, which has certainly been a factor in the elimination of forms, although it played no role in producing any qualities or irritabilities. The fact that cases of tropism occur even where they are of no use, shows how the play of the blind forces of na- ture can result in purposeful mechanisms. There is only one way by which such purposeful mechanisms can originate in nature; namely, by the existence in excess of the elements that must meet in order to bring them about. In green plants and in some animals the positive heliotropism is useful; yet there exists probably an endless number of heliotropic animals for which their heliotropism is about as useless as is galvanotropism. The prerequisites for heliotropism are a symmetrical body form, which seems to be present in almost all organ- isms— although some asymmetries exist — and the presence of photo- sensitive substances, which is not quite so common, but certainly not infrequent. Some of the regular substances found in protoplasm seem to turn readily into a photosensitive form. As the two conditions men- tioned above are quite common, the laws of probability make it neces- sary that in a certain number of cases both conditions will be fulfilled, and then we may expect heliotropic actions. If it now occurs that in an organism the turning to the light helps it to find its food, as is the case with certain caterpillars, e.g. Porthesia chrysorrhcea, or the stems of green plants whose starch is manufactured by light, we have a "purpose- ful mechanism." Again, according to the laws of probability, the number of animals in which the three groups of conditions meet is much smaller than where only two meet. The tropisms thus furnish an insight into the origin of purposeful reactions by the blind forces of nature.

IT is comparatively easy for the physicist to give to his data the form of a mathematical law, inasmuch as the independent variables are mostly in evidence, and all that remains to be done is to find the formula which expresses the relation between the variable and the function. In biology the independent variable is generally unknown, and the main energy of the investigator must be devoted to discovering this variable. The history of the problem of fertilization is extremely instructive in this regard. Although the fact that many animals, e.g. fishes, birds, etc., develop from an egg has been known as long as man has observed, it was not until 1827 that von Baer discovered the mammalian egg; and although Leuwenhoek, or a pupil of his, discovered the existence of spermatozoa in the sperm as early as 1677, it was not until 1843 that the fact was really established that generally the development of the egg is caused by the entrance of a spermatozoon.

As far as we know at present the entrance of a spermatozoon into the egg has two kinds of effects which must be kept apart : the first, namely, the starting of the process of development, the developmental effect; the second, the transmission of the paternal qualities to the new organism, the hereditary effect. We shall first discuss the develop- mental effects of the spermatozoon upon the egg, raising the question whether this effect of the spermatozoon is specific or general ; that is to say, whether a spermatozoon can cause only the development of an egg of the same species or of any egg. It is well known that animals belong- ing to the same family, e.g. various kinds of dogs, the horse and the donkey, can be successfully crossed. In fishes, also, it has long been known that various types of hybrids can be easily obtained. Spallan- zani and other observers were never able to obtain hybrid larvae among the Batrachians. Pfliiger, however, found that the first segmentations can be produced in the eggs of Rana jusca by the sperm of a salamander

(Triton alpestris}. When the eggs of the toad (Bujo vulgaris] are fer- tilized with the sperm of Rana jusca, they develop beyond the morula stage.* Born hybridized various kinds of toads. t It is rather remarkable that all these experiments seemed to indicate that the fertilizing power of a spermatozoon is quite specific, and that it does not go beyond the closely related forms. It was of considerable interest to find out whether the stimulating power of a spermatozoon might not be extended to more distant species. Nobody had succeeded in fertilizing the eggs of the sea urchin with the sperm of the starfish, and I had myself vainly tried to accomplish this result until it occurred to me that by altering the constitution of the sea water this result might be accomplished. The sea water has normally a practically neutral reaction. If, however, just enough NaHO or Na2CO3 is added to make its reaction faintly alkaline, the eggs of the sea urchin, Strongylo- centrotus purpuratus, can be fertilized by the sperm of every starfish which has thus far been tried, and by that of Ophiurians.% It suffices

for this purpose to add i to 2 c.c. — NaHO to 100 c.c. of sea water. The relative number of sea-urchin eggs that can be fertilized in this way by the sperm of starfish or brittle star varies for various forms. With the sperm of Asterias ochracea, Aster las capitata, and an Ophiurian, as many as 50 per cent of the eggs could be fertilized, while with the sperm of the twenty ray starfish (Pycnopodia spuria) only 5 per cent, and with the sperm of Asterina only i per cent. In normal sea water only ex- ceptionally an egg of Strongylocentrotus is fertilized by the sperm of Asterias; and in this case the- fertilization occurs very late, --from twelve to thirty-six hours after the sperm has been added. The sperm of Pycnopodia and Asterina was never able to cause a fertilization of the sea urchin's egg in normal sea water.

It seems that the increase in the alkalinity of the sea water increases only the fertilizing power of the spermatozoon, and not that of the egg. When the sperm of starfish is introduced into alkaline sea water in which there are eggs of Strongylocentrotus, it takes from five to eight minutes before the fertilization membrane - - which indicates the entrance of a spermatozoon into the egg - - is formed. After a short time, which varies with the concentration of the HO-ions in the sea water, the sperm loses its fertilizing power, and the spermatozoa agglutinate with each other. The eggs, however, do not .lose their power of being fertilized by remaining in this abnormal solution. If the spermatozoa of the star-

fish are brought from the alkaline sea water into normal sea water which contains the eggs of the sea urchin, none or only a few eggs are fertilized, showing that only in the alkaline sea water does the sperm of the starfish possess the qualities necessary for the fertilization of the egg of the sea urchin. It is not so easy to decide which change must occur in the sperm of the starfish in order to enable it to fertilize the egg of the sea urchin. It is certain that the addition of alkali increases the energy of the mo- tions of the spermatozoa of the starfish, but it is also certain that the addition of bicarbonate to sea water -brings about an equal or a still more powerful increase in the energy of the motions of the spermatozoa of the starfish without increasing their power of fertilizing the eggs of the sea urchin. At present it is generally assumed that all that is neces- sary for 'the entrance of the spermatozoon into the egg is the ciliary motion of the spermatozoon which brings it in contact with the egg, and that the entrance of the spermatozoon into the protoplasm of the egg is due to the energy of its ciliary motion. I consider it possible on the basis of these observations that the ciliary motion of the spermato- zoon is required only to bring spermatozoon and egg protoplasm into close contact, and that the entrance of the spermatozoon into the inte- rior of the egg protoplasm is due to surface tension forces. It is not impossible that the conditions for this process depend upon the sur- face tension between spermatozoon and sea water becoming greater than the sum of surface tensions between sea water and egg, and sper- matozoon and egg. In this case the egg protoplasm must spread at the limit between spermatozoon and sea water. The spermatozoon is thus introduced into the interior of the egg. These ideas are supported by the fact that the spermatozoon of the starfish fertilizes the eggs of its own species in normal sea water, and that the process is not aided by making the sea water alkaline.

It is also hardly necessary to mention the fact that the eggs of Strongy- locentrotus purpuratus can be best fertilized in neutral sea water, not in alkaline sea water. It is a surprising fact that in the alkaline sea water in which the fertilization of the sea urchin's egg by starfish sperm succeeds best, the fertilization of the same egg by sperm of their own species is rendered difficult or impossible. This may be due to the fact that the motility of the spermatozoa of the sea urchin is diminished by the alka- line sea water.

I have tried to fertilize the eggs of the sea urchin with the sperm of Annelids and Mollusks, but thus far without success. It therefore looks as if the fertilizing power of a spermatozoon were to some extent at least specific. It is also possible, however, that if our idea concerning the role of surface tension for the entrance of the spermatozoon is cor- rect, these restrictions to the fertilizing power of the spermatozoon are only apparent, and that we have only to find modifications of the natu- ral media, which allow the spermatozoon to enter the eggs of foreign species.

As a rule only one spermatozoon enters an egg: as soon as this has entered no further spermatozoon can enter. This is also true for frag- ments of an egg. First, O. and R. Hertwig and later Boveri, Delage, and many other authors showed that a piece of an unfertilized egg can be fertilized by a spermatozoon. . Janssens has recently observed that if a piece of protoplasm be cut off from a fertilized egg, this can no longer be fertilized. It is not impossible that the entrance of a spermatozoon alters the surface tension of the protoplasm of the egg, making it thus impossible for another spermatozoon to enter.

The egg of a starfish is, as a rule, not yet ripe, i.e. capable of being entered by a spermatozoon immediately after it is taken from the ovary. It has to lie for about two hours in sea water before it is ready for fertili- zation. During this time the polar bodies are thrown out. Delage has shown that if a piece of protoplasm be cut off from an egg of a starfish (Asterias glacial-is] before it is ripe, it cannot be fertilized by a sperma- tozoon, but that this can be done when the piece of protoplasm is cut off from the egg after the egg has gone through the process of maturation.*

It is generally stated that the pollen of a hermaphroditic plant can- not fertilize the egg cells of the same individual. Castle found that similar though less pronounced conditions exist in a hermaphroditic Ascidian ; namely, Ciona intestinalis. The eggs of a Ciona can, as a rule, not be fertilized with the sperm of the same individual, while they can be fertilized with the sperm of another individual. This immunity of the eggs against sperm of the same individual is not without exception. In some cases Castle found that 5, 10, or even 50 per cent of the eggs of an individual could be fertilized with sperm of the same individual. Morgan confirmed Castle's observations, and found that if the eggs are put for about ten minutes in a 2 per cent ether solution in sea water, in a number of (but not in all) cases the number of fertilized eggs shows a sliht increase.

2. ARTIFICIAL PARTHENOGENESIS AND THE THEORY OF FERTILIZATION It is hardly necessary to state that at all times authors have been ready to explain the fertilizing or developmental action of the sper- matozob'n. One such explanation states that the spermatozoon im- parts a peculiar mode of motion to the egg, leaving it to science to find out what this mysterious motion is. Other authors say that the egg is comparable to a watch which cannot go unless a spermatozoon enters, leaving it to science to find out the wheels and the spring in the egg, and the relation of the spermatozoon to this mechanism. Others again say that the spermatozoon exercises a stimulus, forgetting, however, to tell us what is the nature of the stimulus. The list of such expla- nations might be continued, but they all show the same characteristic; namely, that an explanation by phrases or words is offered where an explanation by facts is wanted. Instead of devoting any time to this kind of metaphysics, we shall consider some of the facts of par- thenogenesis.

, The oldest and best-known case of parthenogenesis is that of plant lice (Aphides}. When the temperature and moisture are suffi- ciently high, the Aphides reproduce themselves parthenogenetically. Males do not exist under such circumstances. This condition can be maintained for years, possibly indefinitely. Similar cases of partheno- genesis seem to occur in Daphnia. A remarkable case of parthenoge- netic development occurs among bees (and possibly among social wasps), where, according to Dzierzon, the male bees originate from unfertilized eggs, while the female (queens and workers) originate from fertilized eggs. The queen pairs only once and the sperm is carried in a recep- tacle. When an egg passes the duct without any sperm coming from the duct, it remains unfertilized. Dzierzon found that old queens lay only eggs from which male bees develop, and the examination of the receptacle showed that in such cases the receptacle was free from sperm. It was, moreover, observed that the workers, whose rudimentary sexual organs exclude copulation, occasionally lay eggs from which, however, only male bees originate. The observations of Dzierzon were confirmed and enlarged upon by Siebold, Leuckart and very recently by Petrun- kewitsch.

In such cases of parthenogenesis the development of the egg is not called forth by a spermatozoon, but by another, at present, unknown condition. More recently the fact has been established that eggs, which naturally develop only when a spermatozoon enters, can be caused to develop artificially by certain physical and chemical means. In 1886 Tichomiroff published the fact that the unfertilized eggs of silkworm, Bombyx mori, can be caused to develop by rubbing them gently with a brush, or by putting them for a short time into concentrated sulphuric acid. Siebold had already mentioned, and Nussbaum confirmed his observation, that a small number of such eggs develop without these

means.* The publication of Tichomiroff caused Dewitz to make simi- lar experiments on the eggs of frogs, and he believed that he found that treatment of these eggs with corrosive sublimate caused them to seg- ment, f Roux, however, showed that Dewitz's conclusion was based upon an error, inasmuch as the eggs did not segment, but underwent coagulation, which gave the surface of the egg occasionally the appear- ance of having segmented. A Russian author, Kulagin, made the statement that he put fish eggs into diphtheria antitoxine and saw a segmentation ; but inasmuch as he published but this one statement on the subject, it is hard to tell whether or not sources of error were sufficiently avoided.

In 1887, O. and R. Hertwig published their well-known experiments on the effects of various poisons on the segmentation of the eggs of Echino- derms. During these experiments, R. Hertwig made the observation that if eggs are transitorily treated with a o.i per cent solution of sul- phate of strychnia, and are then put back into sea water, these eggs show karyokinetic figures, and occasionally segment. This observa- tion was repeatedly discussed by him in subsequent papers.J Hertwig raised the question whether other media might not have similar effects. Mead§ found in Woods Hole, that if a little KC1 is added to sea water, the eggs of Chcetopiorus, a marine Annelid, throw out their polar bodies, a process which in this form is normally only produced by the entrance of a spermatozoon into the egg. NaCl has no such effect. Morgan tried the effect of the addition of NaCl and other salts to sea water on unfertilized eggs of sea urchins, in order to test some statements made by myself and Norman concerning the effects of these salts on fertilized eggs. He found that unfertilized eggs form artificial astrospheres in such solutions, || and afterwardlf made the important observation that if these eggs are put back into normal sea water, they may begin to segment. He states, however, that "the result is a mass of extremely minute granules or pieces. These pieces never acquire cilia and do not produce any form that resembles any stage of the normal embryo. Later the masses disintegrate " (p. 454). The pathological cases of tumors, or galls show also that cell division and growth may be produced which do not lead to the formation of an embryo.

I was led to try experiments on artificial parthenogenesis in order t R. Hertwig, Ueber Befruchtung nnd Conjugation, Verhandl. der deutsch. zoolog. Gesellsch., 1892 ; and Sitziingsber. der Gesclhch. filr Morphologic und Physiologie, in Miinchen, 1895 ; and Festschrift fit >• Gegenbauer, Vol. 2, p. 23, 1896. § A. D. Mead, Lectures Delivered at IVoods Hole, Boston, 1898. to test the idea of the role of ion-proteids in the mechanism of living matter. If it were true that the salts played the role which I was in- clined to ascribe to them, it might also be possible to cause with their aid the normal development of eggs. The experiments did not sustain this idea as I had expected, but I succeeded in producing plutei from the unfertilized egg of the sea urchin by exposing the eggs for about two hours to sea water whose concentration had been raised by about 40 per cent to 50 per cent. It was immaterial which substance was used to raise me concentration of the sea water, except for the fact that no substances could be used which injured the eggs too much. The best effects can be produced by raising the concentration of the sea water through the addition of NaCl.*

When unfertilized eggs are put into hypertonic sea water, they lose water and shrink. When put back into normal sea water, they absorb water again. We must therefore raise the question as to which of these two conditions causes the egg to develop, the loss of water when the egg is put into the concentrated sea water, or the taking up of water when it is put back into normal sea water. It can be shown that the former is the cause. If we increase the concentration of the sea water less than 40 per cent, if e.g. we add 7 c.c. of a 2 J m solution of NaCl to 93 c.c. of sea water, some of the unfertilized eggs of Arbacia will develop into swimming blastula?, even if left permanently in the hypertonic sea water. I have recently repeated this experiment with the eggs of Strongylocen- trotus. If the eggs of this sea urchin were left in a mixture of 100 c.c. sea water + 5 c.c. i\ n NaCl solution, after about six hours segmenta- tion began, and after one or two days swimming larvae began to appear. These larvae, however, did not develop into gastrulas or plutei, probably on account of the abnormal condition of the sea water. In this case only a loss, but no taking up, of water occurred. When the unfertilized eggs of the sea urchin are put permanently or transitorily into sea water which is diluted with distilled water, no development is produced.

But although the osmotic method led to the development of larvae from the egg, it differed in a number of points in its effects from the pro- cess of fertilization by spermatozoa. In the first place, the eggs fertilized with sperm form a characteristic membrane as soon as the spermatozoon has entered, while the unfertilized eggs treated with hypertonic sea water develop without the formation of a membrane. Second, the rate of development is considerably faster in the fertilized egg than in the egg caused to develop parthenogenetically. Third, the larvae originating from fertilized eggs rise to the surface of the water as soon as they begin

to swim, while those originating by the above-mentioned osmotic pro- cess swim at the bottom of the dish. Fourth, the number of larvae developing from fertilized eggs is, as a rule, practically 100 per cent, while in the case of artificial parthenogenesis a much smaller percentage of the eggs develop into swimming larvae. In the case of Arbacia, I often succeeded in causing more than 20 per cent of the unfertilized eggs to develop, but in the case of Strongylocentrotus - - the form of the sea urchin common at Pacific Grove — I was rarely able to obtain even as high a percentage of developing eggs. Often enough only a fraction of i per cent of the eggs yielded swimming larvae by the osmotic method of artificial parthenogenesis.

In thinking over the possible cause of this difference between the development of the egg fertilized by sperm and of the egg caused to develop by osmotic influences, it occurred to me that the spermatozoon might carry into the egg not one, but several, substances or conditions, each of which was responsible for only a part of the specific features of sexual fertilization; and that in order to completely imitate the action of the spermatozoon it might be necessary to combine two methods of artificial parthenogenesis, each of which alone imitated the process of sexual fertilization only partially. This latter idea proved correct far beyond my expectations.*

I found that if the eggs of Strongylocentrotus purpuratus are put into propionic, butyric or valerianic acid, are added, and are left in this water for from one half to one and one half minutes, they form a mem- brane when put back into normal sea water. The eggs go through the internal changes characteristic of nuclear division, but they rarely seg- ment. In about six hours they begin to disintegrate, and after twenty- four hours scarcely an egg is left alive. If the eggs are left in the acidulated sea water, they neither form a membrane nor segment. If the eggs which have formed a membrane are put for from twenty-five to fifty minutes into sea water whose concentration has been raised by adding 15 c.c. 2\ n NaCl solution to 100 c.c. of sea water, the results are surprising. Instead of a fraction of i per cent of the eggs develop- ing, I had it in my power to cause 90 to 100 per cent of the eggs to develop. All the eggs formed a membrane which is characteristic of the egg fertilized with sperm. The rate of segmentation was practically the same as that of the eggs of the same female fertilized with sperm. A large percentage of the blastulae originating from this combination of methods looked perfectly normal, and rose to the surface of the sea

water. Their further development into gastrulas and plutei occurred with the same velocity as that of the control eggs, which had been fer- tilized by sperm; and the larvae showed an equal degree of vitality. It is an easy matter to produce and collect an unlimited number of plutei from the eggs treated with this method. When the eggs are taken out too early from the acidulated sea water, they form no membrane, and the same is true when they remain too long in the acidulated sea water. If eggs that have been treated with a fatty acid without forming a membrane are submitted to the hypertonic sea water for from twenty-five to fifty minutes, they will not develop into larvae, and not even segment. It is therefore obvious that the membrane formation and not the treatment with acid is responsible for these effects. This is corroborated by some further observations.