(_After Wilson._)
A Gastrula from a whole egg; B, C and D, gastrulae from single cells artificially separated, (B) from the two-celled stage, (C) from the four-celled, and (D) from the eight-celled stages of normal development.]
From one of the first two segmentation spheres of an echinoid egg, Driesch was able to rear successive embryonic stages (_Gastrula_ and _Pluteus_), which were normal in shape, but one-half the usual size. Wilson's results, obtained by shaking apart the segmentation spheres, were even more interesting, as they were performed upon amphioxus, a more highly-organized animal. He reared gastrulae and older embryos with notochord and nerve-tube, which were perfect and normal, except in size. They were one-half, one-quarter, or one-eighth of the usual size, according as they were reared from cells isolated from the two, four, or eight-celled stage of the segmenting egg.
Results which Chabry and I gained by destroying, by puncture, one of the first two segmentation spheres, assist the present argument. Although one-half of the mass had been destroyed, Chabry obtained, in the case of an ascidian, and I obtained, in the common frog, embryos with notochord and nerve-plate. These developed directly and normally, although, in the case of the frog, there was a slight defect at the ventral posterior part of the body, where the arrested protoplasmic mass came to lie.
All these experiments show that the first two (and in some cases the first four) results of division can assume a quite different bearing as regards their function in the mechanical building of the embryo, according to whether they remain bound with each other into a whole or are separated and develop by themselves. In the former case, each forms only one-half (in some cases only a fourth) of the whole. In the latter case, each by itself produces the whole. The half and the whole, then, of the first cleavage-cells are identical in real nature, and, according to the circumstances, can develop, now in this way, now in that.
Even if Weismann were to admit the correctness of these experiments, perhaps he would not consider that they contradicted his theory of the germplasm and the segregation of the hereditary mass, but would make a supplemental hypothesis, which, from the spirit of his theory, could be none other than this: each of the first cleavage-cells, in addition to its specific part of the hereditary mass, the part that controls its normal course of development, possesses an accessory idioplasm, an undivided fragment of the germplasm, left behind to be ready for unforeseen emergencies; this part takes command when, in consequence of violence, a separated part develops into the whole.
But such an assumption does not go far enough, if it be confined to the first cleavage-cells. By compression of the frog's egg, I have shown that the pole passing through the blastopore, which coincides with the chief axis of the future embryo, may assume different relations to the first segmentation-plane, sometimes coinciding with that, sometimes making a right or an acute angle with it. It is clear that in each of these cases the embryonal-cells take a different share in the formation of the regions of the body, and that they must be fore-endowed with the capacity of playing different parts.
The developmental history of double monsters enforces the same doctrine; such are common among the embryos of fish, and rather less common among chicks. From causes of which we are ignorant two, instead of one, gastrula stages may arise at separate regions of the germinal layer of the egg.
According to the position of these two invaginations, which may be regarded as crystallisation-points for the formation of the future embryo, the cells of the germinal disc will be drawn into the process of development, and, falling into groups, will build up organs. In relation to this double gastrulation, there may arise, for instance, four instead of two primitive ears, eyes, and nasal organs; and these arise from cell-groups, the choice of which is determined by their relation to the position of the gastrula-invagination.
From various other experiments, conducted so as to distort the normal course of development, I have obtained parallel results.
Taking frogs' eggs immediately after fertilisation, I compressed them strongly between parallel, horizontally placed glass plates. I then inverted them, so that the vegetative pole came to lie uppermost. In spite of their unnatural relation to gravity, they developed further, and became abnormal, quite unsymmetrical embryos.
In another experiment, taking a triton's eggs after they had divided into two spheres, I surrounded them with a silk thread in the plane of the first cleavage, and tightened the thread until the embryo assumed the form of a sand-glass. The deformity of the resulting larvae was very different, and perhaps depended on the tightness of the constriction. Some became greatly elongated, and had developed so that the thread surrounded the dorsal nerve-cord. In other cases the dorsally-placed organs arose only from one-half of the sand-glass-shaped embryo, while the other half gave rise to the ventral part of the body. In this case the dorsal organs (nerve-tube and notochord) were doubled over like a snare, the head and tail ends, the mouth and the region of the anus, being bent in at the position of the constricting thread.
The important point is that in both the experiments, in the case of the frog and of the triton, the cell-material, separated at the first cleavage, was turned to a use quite different to its use in the formation of a normal embryo.
We may conclude with a very convincing proof. In the above-mentioned abnormal development of the frog's egg it happened that one edge of the blastopore, on account of its weight, was very much bent outwards. In consequence of this the cleft of the blastopore lay between the normal blastopore-lip and the everted border of the other lip. When the notochord and the nerve-plate appeared, as a result of this abnormal condition, they grew from a cell-material that was quite different to that which gives them origin in normal cases.[15]
In these cases Weismann cannot apply his accessory conception, the existence of supplementary idioplasm, only to the nuclei arising from the first division; he must extend it to the thousands of embryonic cells that arise by division up to the time for the appearance of the nerve-tube and notochord. The behaviour of these cells under fortuitously changed conditions shows them all to be endowed with the capacity of development in different directions.
FIFTH GROUP OF FACTS.--PHENOMENA OF VEGETATIVE AFFINITY.[16]
Many considerations, taken from the region of general physiology, support the view that all the cells of an individual, of any species, are alike, and are to be distinguished from one another only by the special development of one character.
Formerly, indeed, many biologists, relying upon the optical appearances presented in microscopical investigation, have been inclined to the view that the visible qualities of a tissue, as revealed by the microscope, were the only, or the chief, distinctive characters. For instance, by microscopical investigation one cannot distinguish the tendons, nerves, bones, and cartilages of a dog from the corresponding tissues in a horse.
So far as their special use in the organism goes, one might interchange the corresponding parts in these two mammals. A tendon from the dog, if large enough, might be attached to the muscle of a horse, and would transmit the pull of the muscle on the bone just as well, and would completely satisfy the mechanical duties of the horse's tendon. The same might happen in the case of a bone, of a cartilage, or of a nerve-fibre.
As a matter of fact, the idea that parts of the tissues of different animals may serve to replace one another has been employed repeatedly in science, especially in the science of medicine. But I believe that our ideas are not yet clear upon the matter. The erroneous impression to which I have alluded has arisen because we do not bear in mind that each tissue, each part of an organ, each cell, possesses, in addition to its obvious characters, very many characters that are invisible to us. Such characters are inherent in the tissue-cells because these are parts of a definite organism. In consequence of their specific tissue characters, which are visible to us, we assign cells their place in histological classification; in contrast, we may denote the other characters as constitutional, or species, characters.
No doubt tissue cells are in the same case as genital cells. So far as microscopical characters go, egg cells and spermatozoa are wonderfully alike in all the mammalia; in many cases we could not distinguish between those of different animals. But, because they bear the specific characters, we cannot doubt but that they are as distinct as are the species, although invisibly to us.
The products of the sexual cells show us clearly enough that out of each kind of egg only its own species of organism can be developed. Certainly it is not so plain that, besides their visible microscopical characters, the tissues and organic parts are in possession of more general characters, identical in all the differently-specialised tissues of a single organism; but we may infer the existence of such latent characters, at least partly, from the results obtained, in the case of plants, by grafting, in the case of animals, by transplantation and transfusion.
In the case of plants one may graft a twig cut from one tree upon the stem or lower part of another tree of the same kind, and so bring about a firm and lasting union between the two. In a short time the corresponding tissues of the parts brought into connection quietly unite. Thus from two different individuals a single living organism may be produced artificially.
One would expect, therefore, that a twig and stem, chosen from two closely allied species, such as, for instance, the pear and the apple, would unite when the suitable tissues were put together. But this does not happen.
Successful grafting depends far less on the conjunction of obviously appropriate parts than upon characters unrecognisable by us, such as deep-seated kinship between the parts, and the specific characters of their cells; while in the case of individuals of the same species two pieces will unite even if they are not brought together in appropriate conjunction, or when they belong to different parts of the organism, as, for instance, to the root and the leaf; yet in the absence of deep-seated kinship union will not take place.
Generally this kinship, which has been called vegetative affinity, depends, like sexual affinity, upon the degree of systematic relationship. It appears that the same condition of things occurs as when, in ordinary fertilisation, sexual cells from different varieties, or species, are united. In both cases it happens, on the average, that union is the more to be expected the more closely the plants concerned are akin, in a natural system of classification.
But in grafting, as in cross-fertilisation, unexpected exceptions to this rule occur. Relying upon these, Naegeli thought that the external distinguishing tokens do not always indicate correctly the intrinsic constitutional differences. Frequently union will not take place between plants most near akin in classification, most alike in external characters; while it will occur between plants most different in outward aspect and belonging to different genera or even families. In other words, external characters give no certain index to the degree of vegetative affinity or of sexual affinity between two kinds of plants.
As an example of this, Vochting, in his treatise upon transplantation of plant-tissues, takes the tribes of pear-trees. Grafting between these and apple-trees takes place only with difficulty, although the apple is a close kinsman and belongs to the same genus. On the other hand, most of them graft easily upon the quince, although that belongs to a different genus.
In this case, also, there is no sexual affinity between the pollen and the ova. Hybrids are not formed between the pear and the apple.
It seems probable to me, although as yet I cannot get complete proof of it, that sexual and vegetative affinity, that is to say, the relationship between the egg-cell and the pollen of two species, and the relation between twig and stem, depend upon the same intrinsic qualities of that elementary organism the cell.
Vochting distinguishes as harmonic or disharmonic the modes of union between twig and stem, according to whether or no they reach the formation of functional unity. Among cases of disharmony there are several interesting gradations. Generally speaking, in the case of plants not adapted to each other, no attempt at union occurs, and the grafted twig speedily perishes; sometimes even the stem dies, as if it had been poisoned by the graft. In other cases the disharmony is not shown so strongly. The twig and the stem begin to unite, but, sooner or later, disturbances occur, and complete destruction results. According to Vochting, in the case of some _Cruciferae_ the disturbances are as follows: the twig begins to form roots at its lower end, and these grow into the stem of the host. Through them the twig uses as food the juices and salts of the stem, refusing to unite with the stem so as to form a single individual. As Vochting says, this formation of roots simply is an attempt on the part of the twig to complete its own individuality. Instead of growing into corporate union with the stem, the twig attempts to become a parasite upon it. A further consequence often is, that the stem, too, begins to respond to the unadaptive stranger's influence. Thus, when Vochting grafted a _Rhipsalis paradoxa_ on an _Opuntia labouretiana_, he found that round the roots of the graft the tissues of the host threw out a protective sheath of cork, or turned in places to a gelatinous mass.
In some cases experimenters have overcome disharmony between two species, A and B, by making use of a third species, C, with a vegetative affinity for both A and B. Thus, an intermediary between the two disharmonic forms is made, and by such an arrangement a single functional individual is produced from pieces of three different species. Thus, upon A, as stock, a shoot of C is grafted, while upon this shoot of C, as stock, a shoot of B in turn is grafted.
In the matter of these different grades of disharmony, a comparison may be made between sexual and vegetative affinities. In many cases the spermatozoa of one species will not impregnate the eggs of another species.
In other cases, the alien spermatozoon may penetrate the egg and unite with its nucleus, making, however, an unsatisfactory combination in various degrees of infertility. Sometimes the fertilised egg divides only a few times and then dies; sometimes development proceeds to the stage of the blastula, the gastrula, or even further; but it then comes to an end, through intrinsic causes beyond our ken, and, finally, complete destruction follows.
Our acquaintance with what happens in transplantation of animal tissues is smaller than in the sphere of botany.
Long ago, Trembley attempted to cause, by grafting, the union of two pieces of hydroid polyps into a single individual. He divided, across their middles, two specimens of _Hydra fusca_, and then, in a watch-glass, applied the upper end of one to the lower end of the other. In one case he was rewarded by the occurrence of complete union; for, after a few days, on feeding the upper end with a worm, it was passed on into the lower end.
Later on buds arose, both above and below the point of union. Trembley, however, was unable to graft on each other parts of different species, parts of the green hydra, _Hydra viridis_, upon the common hydra.
Transplantations of single tissues or organs have been made more often, and by several investigators. I shall mention only the older results of Ollier and M. Bert, and those made in 1893 by A. Schmitt and Beresowsky.
Ollier exposed the bone of an animal, and, carefully removing a part of the periosteum, planted it in the connective tissue under the skin in another part of the body. The consequences differed according as the transplanted tissue was imbedded in another animal of the same species, or of another species. In the first case the piece of periosteum grew, obtaining a supply of blood from vessels which grew out into it from the surrounding connective tissue in which it was embedded. In a short time lamellae of bone were formed by the layer of osteoblasts, so that a small plate of bone was formed under the skin. This, however, proved always but a temporary structure, for, being formed in an inappropriate spot, and, therefore, being functionless, it was soon reabsorbed. In the second case, however, in which the piece of periosteum was removed from the bone of a dog and planted in a cat, rabbit, goat, camel, or fowl (or _vice versa_), formation of bone did not occur; either the piece of periosteum was absorbed, or set up suppuration around it, or became enclosed in a cyst.
Paul Bert's experiments were the following. He removed pieces two or three centimetres long from the tails of white rats a few days old, skinned each piece, and planted it in the connective tissue under the skin of the same animal. In a few days circulation of blood was established in the pieces of the tails, by union with vessels from the connective tissue in which they were embedded. Muscles and nerves degenerated, but the other tissues, bones, cartilages, and connective tissue, grew vigorously, so that, in animals killed and examined a month after the operation, the pieces of tail, implanted when they were two or three centimetres long, had grown five to nine centimetres long.
The result was totally different when the transplantation was made from one species to another. When the tip of the tail of a _Mus decumanus_ or a _Mus rattus_ was transplanted to a squirrel, guinea-pig, rabbit, cat, dog (or _vice versa_), either extensive suppuration took place, and the piece was extruded, while sometimes the subject of the experiment died; or, after a less turbulent course, the alien piece was absorbed. The continuance of life and growth in the piece only took place when the two animals concerned were allied very closely. Thus success followed transplantation from _Mus rattus_ to _Mus decumanus_ (or _vice versa_), but not when it was from _Mus sylvaticus_ to _Mus rattus_.
The recent experiments of A. Schmitt and Beresowsky lead to the same conclusion. The former succeeded in making pieces of living bone 'take'
only when the transplantation was from one individual to another of the same species, or to another part of the same individual. Beresowsky transplanted pieces of frog's skin to the dog and the guinea-pig, and pieces of dog's skin to the guinea-pig, and always found that they died, or were thrust out as foreign bodies.
Precisely the same results follow transfusion of blood between animals of different species. There is complete agreement among investigators. When the blood is made to flow directly from the vessels of one animal to the vessels of an animal of a different species, as from the dog to rabbit, or from dog to sheep (or _vice versa_); or when it has been first freed from fibrin and then injected, the result is always the same. 'We have always found,' says Ponfick, summing up the results of the investigation, 'not only that blood of another species acts in strong doses as a poison, and in weaker or smaller doses is harmful, but that (and this seems to me my most important result) in every case the blood-corpuscles are destroyed almost completely, probably quite completely.' In a very few minutes, in the case of disharmonic kinds of blood, the red corpuscles degenerate, and the haemoglobin, becoming dissolved in the blood-plasma, soon appears in the urine. In the case of transfusion of similar blood between individuals of the same or of very closely related species, the haemoglobin does not appear in the urine except after very large doses; and Ponfick infers that the red blood-corpuscles, either all of them or most of them, remain unchanged in the new animal.
Landois has carried out transfusion between the remotest species, between different families of mammals, and between mammals, birds, and amphibia; from these he drew 'the inference, important for classification of animals, that those animals anatomically most nearly allied have their blood most closely alike.' In fact, 'the destruction of the foreign blood happens the more slowly the more nearly the animals are allied.' 'Thus, in doubtful cases, experiments on transfusion might settle degrees of relationship.
Between individuals of the same species transfusion is a complete success; when the species are closely allied, the transfused blood disappears only very gradually, and large quantities may be transfused without harm. The further apart the animals may be, in a system of classification, the more violently the destruction of the foreign blood takes place, and the smaller is the quantity that can be endured in the vessels. Thus, in the extent to which blood transfusion may occur, I see a step towards the foundation of a Darwinian theory applied to cells.'
As yet, transplantations and transfusions between animals of different species have been considered with a view to their importance in surgery and in medicine, rather than from their purely physiological side. From the results given above, in which I believe, although there might be drawn from literature contradictory results--in which, however, I cannot feel confident--I am prepared to extend a conclusion to the animal kingdom that is better supported in botany: the conclusion that the cells and tissues possess, in addition to their definite microscopical characters, more general, intrinsic, specific characters, and, that one may speak of the vegetative affinities between tissues exactly as one speaks of the sexual affinities between reproductive cells.
SUMMARY OF THE CONCLUSIONS IN THE FIRST SECTION.
Summing up what has been said in the preceding pages, we find a large series of facts supporting our contention that cells multiply only by doubling division. First comes the fundamental circumstance that single-celled organisms exhibit only doubling division, as by that alone the permanence of species, which experience shows us to exist, is possible.
Secondly, some facts of reproduction were considered. The formation of germinal tissues, and, in the case of lower plants and animals, the occurrence of budding in almost any part of the body, are easily intelligible if every cell, like the egg-cell, has been formed by doubling division, and so contains the rudiments of all parts of the organism; and if thus, on the call of special conditions, every cell may become a germ-cell again.
Thirdly, great stress is to be laid on those experiments in which the process of development was interfered with at different stages, as these showed that the separate cells which arose by division were not predestined unalterably for a particular _role_, according to a predetermined plan (facts of regeneration and heteromorphosis).
Fourthly, the results of grafting, transplantation, and transfusion indicate that the cells and tissues of an organism possess, in addition to their patent microscopical characters, latent characters, which show themselves to be peculiar to the species.
How does Weismann attempt to reconcile his hypothesis of differentiating division with these facts? By the provision of different complementary hypotheses, which, as we have seen, amount to this, that he allows the set of rudiments which he had turned out by differentiating division of the cell to creep in again by a back-door. He accomplishes this by his idea that the germplasm may undergo, simultaneously, doubling and differentiating division. In these cases cell-division has a double aspect.