In the first place the idioplasm does not form a directly continuous network throughout the entire body; and, secondly, the whole organism is not penetrated by a single substance of homogeneous constitution, but each special kind of cell must contain the specific idioplasm or nucleoplasm which determines its nature. There are therefore in each organism a multitude of different kinds of idioplasm. Thus we should be quite justified in generally speaking of Nägeli’s idioplasm as nucleoplasm, and vice versa.
It is perfectly certain that the idioplasm cannot form a continuous network through the whole organism, if it is seated in the nucleus and not in the cell-body. Even if the bodies of cells are everywhere connected by fine processes (as has been proved in animals by Leydig and Heitzmann, and in plants by various botanists), they do not form a network of idioplasm but of somato-plasm; a substance which, according to Nägeli, stands in marked contrast to idioplasm. Strasburger has indeed already spoken of a ‘cyto-idioplasm,’ and it is certainly obvious that the cell-body often possesses a specific character, but we must in all cases assume that such a character is impressed upon it by the influence of the nucleus, or, in other words, that the direction in which the cell-substance is differentiated in the course of development is determined by the quality of its nuclear substance. So far, therefore, the determining nuclear substance corresponds to the idioplasm alone, while the substance of the cell-body must be identified with the somato-plasm (‘Nährplasma’) of Nägeli. At all events, in practice, it will be well to restrict the term idioplasm to the regulative nuclear substance alone, if we desire to retain the well-chosen terms of Nägeli’s theory.
But the second part of Nägeli’s theory of the idioplasm is also untenable. It is impossible that this substance can have the same constitution everywhere in the organism and during every stage of its ontogeny. If this were so, how could the idioplasm effect the great differences which obtain in the formation of the various parts of the organism? In some passages of his work Nägeli seems to express the same opinion; e. g. on page 31 he says, ‘It would be practicable to regard—although only in a metaphorical sense—the idioplasms of the different cells of an individual as themselves different, inasmuch as they possess specific powers of production: we should thus include among these idioplasms all the conditions of the organism which bring about the display of specific activity on the part of cells.’ It can be clearly seen from the passages immediately preceding and succeeding the above-quoted sentence, that Nägeli, in speaking of these changes in the idioplasm, does not refer to material, but only to dynamical changes. On page 53 he lays special stress upon the statement that ‘the idioplasm during its growth retains its specific constitution everywhere throughout the organism,’ and it is only ‘within these fixed structural limits that it changes its conditions of tension and movement, and thus alters the forms of growth and activity which are possible at each time and place.’ Against such an interpretation weighty objections can be raised. At present I will only mention that the meaning of the phrase ‘conditions of tension and movement’ ought to be made clear, and that we ought to be informed how it is that mere differences in tension can produce as many different effects as could have been produced by differences of constitution. If any one were to assert that in Daphnidae, or in any other forms which produce two kinds of eggs, the power of developing only after a period of rest, possessed by the winter-eggs, is based upon the fact that their idioplasm is identical with that of the summer-eggs, but is in another condition of tension, I should think such a hypothesis would be well worth consideration, for the animals which arise from the winter-eggs are identical with those produced in summer: the idioplasm which caused their formation must therefore be identical in its constitution; and can only differ in the two cases, as water differs from ice. But the case is quite otherwise in the stages of ontogeny. How many different conditions of tension ought to be possessed by one and the same idioplasm in order to correspond to the thousand different structures and differentiations of cells in one of the higher organisms? In fact it would be hardly possible to form even an approximate conception of an explanation based upon mere ‘conditions of tensions and movement.’ But, furthermore, difference in effect should correspond, at any rate to some extent, with difference in cause: thus the idioplasm of a muscle-cell ought to differ more from that of a nerve-cell and of a digestive-cell in the same individual, than the idioplasm of the germ-cell of one individual differs from that of other individuals of the same species; and yet, according to Nägeli, the latter small difference in the effect is supposed to be due to difference of quality in the cause—the idioplasm, while the former fundamental difference in the histological differentiation of cells is supposed to follow from mere difference ‘of tension and movement.’
Nägeli’s hypothesis appears to be self-contradictory; for, although its author recognizes the truth of the fundamental law of development, and explains the stages of ontogeny as an abbreviated recapitulation of phyletic stages, he nevertheless explains the latter by a different principle from that which he employs to explain the former. According to Nägeli, the stages of phylogeny are based upon true qualitative differences in the idioplasm: the germ-plasm of a worm is qualitatively different from that of Amphioxus, a frog, or a mammal. But if such phyletic stages occur crowded together in the ontogeny of a single species, they are said to be based upon different ‘conditions of tension and movement’ of one and the same idioplasm! It seems to me to be necessary to conclude that if the idioplasm, in the course of phyletic development, undergoes any alteration in specific constitution, such alterations must also take place in ontogeny; so far at least as the phyletic stages are repeated. Either the whole phyletic development is based upon different ‘conditions of tension and movement,’ or if this—as I believe—is impossible, the stages of ontogeny must be based upon qualitative alterations in the idioplasm.
Involuntarily the question arises—how is it that such an acute thinker fails to perceive this contradiction? But the answer is not far to seek, and Nägeli himself indicates it when he adds these words to the sentence quoted above: ‘It follows therefore that if a cell is detached as a germ-cell in any stage of ontogenetic development, and from any part of the organism, such a cell will contain all the hereditary tendencies of the parent individual.’ In other words, if we are restricted to different ‘conditions of tension and movement’ as an explanation, it seems to follow as a matter of course that the idioplasm can re-assume its original condition, and therefore that the idioplasm of any cell in the body can again become the idioplasm of the germ-cell; for this to take place it is only necessary that the greater tension should become the less, or vice versa. But if we admit a real change in constitution, then the backward development of the idioplasm of the cells of the body into germ-cells appears to be very far from a matter of course, and he who assumes it must bring forward weighty reasons. Nägeli does not produce such reasons, but considers the metamorphosis of the idioplasm in ontogeny as mere differences in the ‘conditions of tension and movement.’ This phrase covers the weak part of his theory; and I look upon it as a valuable proof that Nägeli has also felt that the phenomena of heredity can only find their explanation in the hypothesis of the continuity of the germ-plasm; for his phrase is only capable of obscuring the question as to how the idioplasm of the cells of the body can be re-transformed into the idioplasm of germ-cells.
I am of the opinion that the idioplasm cannot be re-transformed, and I have defended this opinion for some years past[[118]], although I have hitherto laid especial stress on the positive aspect of the question, viz. on the continuity of the germ-plasm. I have attempted to prove that the germ-cells of an organism derive their essential nature from the fact that the germ-plasm of each generation is carried over into that which succeeds it; and I have tried to show that during the development of an egg into an animal, a part of the germ-substance—although only a minute part—passes over unchanged into the organism which is undergoing development, and that this part represents the basis from which future germ-cells arise. In this way it is to a certain extent possible to conceive how it is that the complex molecular structure of the germ-plasm can be retained unchanged, even in its most minute details, through a long series of generations.
But how would this be possible if the germ-plasm were formed anew in each individual by the transformation of somatic idioplasm? And yet if we reject the ‘continuity of the germ-plasm’ we are compelled to adopt this latter hypothesis concerning its origin. It is the hypothesis adopted by Strasburger, and we have therefore to consider how the subject presents itself from his point of view.
I entirely agree with Strasburger when he says, ‘The specific qualities of organisms are based upon nuclei’; and I further agree with him in many of his ideas as to the relation between the nucleus and cell-body: ‘Molecular stimuli proceed from the nucleus into the surrounding cytoplasm; stimuli which, on the one hand, control the phenomena of assimilation in the cell, and, on the other hand, give to the growth of the cytoplasm, which depends upon nutrition, a certain character peculiar to the species.’ ‘The nutritive cytoplasm assimilates, while the nucleus controls the assimilation, and hence the substances assimilated possess a certain constitution and nourish in a certain manner the cyto-idioplasm and the nuclear idioplasm. In this way the cytoplasm takes part in the phenomena of construction, upon which the specific form of the organism depends. This constructive activity of the cyto-idioplasm depends upon the regulative influence of the nuclei.’ The nuclei therefore ‘determine the specific direction in which an organism developes.’
The opinion—derived from the recent study of the phenomena of fertilization—that the nucleus impresses its specific character upon the cell, has received conclusive and important confirmation in the experiments upon the regeneration of Infusoria, conducted simultaneously by M. Nussbaum[[119]] at Bonn, and by A. Gruber[[120]] at Freiburg. Nussbaum’s statement that an artificially separated portion of a Paramaecium, which does not contain any nuclear substance, immediately dies, must not be accepted as of general application, for Gruber has kept similar fragments of other Infusoria alive for several days. Moreover, Gruber had previously shown that individual Protozoa occur, which live in a normal manner, and are yet without a nucleus, although this structure is present in other individuals of the same species. But the meaning of the nucleus is made clear by the fact, published by Gruber, that such artificially separated fragments of Infusoria are incapable of regeneration, while on the other hand those fragments which contain nuclei always regenerate. It is therefore only under the influence of the nucleus that the cell substance re-developes into the full type of the species. In adopting the view that the nucleus is the factor which determines the specific nature of the cell, we stand on a firm foundation upon which we can build with security.
If therefore the first segmentation nucleus contains, in its molecular structure, the whole of the inherited tendencies of development, it must follow that during segmentation and subsequent cell-division, the nucleoplasm will enter upon definite and varied changes which must cause the differences appearing in the cells which are produced; for identical cell-bodies depend, ceteris paribus, upon identical nucleoplasm, and conversely different cells depend upon differences in the nucleoplasm. The fact that the embryo grows more strongly in one direction than in another, that its cell-layers are of different nature and are ultimately differentiated into various organs and tissues,—forces us to accept the conclusion that the nuclear substance has also been changed in nature, and that such changes take place during ontogenetic development in a regular and definite manner. This view is also held by Strasburger, and it must be the opinion of all who seek to derive the development of inherited tendencies from the molecular structure of the germ-plasm, instead of from preformed gemmules.