An Examination of Weismannism

by George John Romanes

I have now furnished as complete a résumé as seems desirable for present purposes of Weismann’s theory of germ-plasm, considered both as a theory of heredity and as a sequent theory of organic evolution. But before proceeding to examine this elaborate system as a whole, I must devote another chapter to a further statement of certain later additions to—and also emendations of—the system as it was originally propounded. These additions and alterations have reference only to the theory of heredity: they do not affect the theory of organic evolution as originally deduced therefrom. Moreover they have all been due to our more recently acquired knowledge touching the morphology and physiology of cell-nuclei: it is for the purpose of bringing his theory of germ-plasm into accord with these results of later researches that Weismann has thus modified the theory as it originally stood. For my own part, I do not see that very much is gained by these newer additions and modifications; but, be this as it may, they are certainly very complicated, and on this account I have thought it best to devote a separate chapter to their consideration. Furthermore, not only in the opinion of Weismann himself, but also in that both of his friends and foes, the main question with which his later essays are concerned—viz., as to whether the nucleus of a cell is the only part of a cell which is concerned in the phenomena of heredity—is regarded as of fundamental importance to his entire edifice. Hence, although I cannot myself perceive that the indisputable importance of this question to any speculations on the subject of heredity is of such special or vital significance to Weismann’s theory, it becomes necessary for me to supply this further chapter for the purpose of presenting the further developments of his theory.

First of all, Weismann has of late years considerably modified his original view touching the relation of germ-cells to body-cells. For while he originally supposed the fundamental distinction in kind to obtain as between the whole contents of a germ-cell and the whole contents of a somatic-cell, he now regards this distinction as obtaining only between the nucleus of a germ-cell and the nucleus of a somatic-cell. In other words, he regards the whole of a germ-cell, with the exception of its nucleus, as resembling the whole of any other cell, with the exception of its nucleus. It is the nucleus of a germ-cell alone that contains germ-plasm: all the rest of such a cell being “nutritive, but not formative.”

This transference of the peculiar or hereditary powers of a germ-cell from the cell as a whole to the nucleus, necessitates certain emendations of the original theory of germ-plasm. In particular, the broad distinction between the whole contents of a germ-cell as “germ-plasm,” and the whole contents of a somatic-cell as “somato-plasm,” is now discarded; and in its stead we have all nuclear matter (whether of germ-cells or somatic-cells) comprised under the one denomination of “nucleo-plasm,” in contradistinction to all the other protoplasm of a cell, which is called “cytoplasm.” Hence Weismann now regards the cytoplasm of a germ-cell as identical with the cytoplasm of all other cells. Its function is merely that of “nourishing” the nucleus, while, on the other hand, it is “controlled” by the nucleus as to its own growth, shape, size, and eventual division.

But it is evident that the nucleo-plasm of a germ-cell must differ from the nucleo-plasm of a somatic-cell, in that it not only “controls” the growth, &c. of its own cell, but likewise presents all the additional characters peculiar to a germ-cell. That is to say, the nucleo-plasm of a germ-cell resembles the nucleo-plasm of a somatic-cell in that it is nourished by, and exercises control over, the cytoplasm of its own particular cell; but it differs from the nucleo-plasm of a somatic-cell in admitting of fertilization, in the capability of reproducing an entire organism, in the endowing of that organism with all its hereditary characters, and, lastly, in providing for its own reproduction in the next generation.

Thus it is evident, as Weismann puts it, that the nucleo-plasm of a germ-cell must be of two kinds—one being concerned with the formation and control of the germ-cell only, while the other has to do with the construction of an entire future organism, and the subsequent reproduction thereof. But not only so; for at each stage in the construction of this future organism, all the somatic-cells, as successively constructed, must likewise contain nucleo-plasm in two kinds—one having to do only with the formation and control of its own individual cell, and the other having to do with the formation of the future somatic-cells, which will have to follow in the course of ontogeny. Therefore, in order to designate this second kind of nucleo-plasm (whether in a germ-cell or a somatic-cell) Weismann borrows from Nägeli the term “idio-plasm[6],” or rather, I should say, he uses the term “nucleo-plasm” when he is speaking of all the contents of a nucleus indiscriminately, while he uses the term “idio-plasm” when he has occasion to speak specially of the two kinds of nucleo-plasm now before us.

Hence, the nuclear contents (nucleo-plasm) of every cell, whether germinal or somatic, present two substances, which we may, in the absence of any better terms supplied by Weismann himself, respectively designate “idio-plasm-A” and “idio-plasm-B.” Idio-plasm-A is the substance which has to do only with the formation and control of the individual cell in which it resides, like a mollusc in its shell. Idio-plasm-B is the substance out of which future cells are to be formed and controlled, when in due course either of ontogeny or phylogeny this idio-plasm-B becomes converted into idio-plasm-A,—i.e., into each subsequently developing tissue or organism, as the case may be. I say ontogeny or phylogeny, and tissue or organism, because, where a germ-cell is concerned, idio-plasm-B is capable of reproducing entire organisms of its own and of subsequent generations; whereas, in the case of all somatic-cells, idio-plasm-B is capable only of reproducing, stage by stage, some greater or less number of the cells which are to construct the single organism of which they form a part. Or, otherwise expressed, in the particular case of a germ-cell idio-plasm-B is germ-plasm, and therefore is alone capable of producing an entire organism of somatic-cells, while it is likewise alone capable of reproducing successive organisms; for it alone contains the carriers of heredity[7].

Thus, idio-plasm-B of an unsegmented germ-nucleus is germ-plasm. But as soon as the germ-nucleus has undergone its first nuclear division, its nucleo-plasm is no longer germ-plasm, inasmuch as each of the half-portions is now no longer capable of reproducing an entire organism—unless it be in the case of identical twins. Similarly in the second nuclear division, each of the four resulting idio-plasms-B is still further removed from the pristine character of germ-plasm; and so on through all successive stages of segmentation. Hence these successive nuclear divisions must indicate a partitioning and re-partitioning of the original idio-plasm-B (germ-plasm) into the idio-plasms-B severally distinctive of all the various cells of the soma.

Now, it is evident that not all the idio-plasm-B of a germ-cell which thus passes over into the nuclei of somatic-cells can be represented by the idio-plasm-B of those cells. At every stage of successive cell-formation a certain part of the original idio-plasm-B of the germ-cell must become the idio-plasm-A of somatic-cells distinctive of that stage. For, supposing that at its differentiation stage 99 the original germ-plasm (now somatic-idio-plasm-B of 99th stage) has reached a phase of ontogeny where the formation of tissue m has next to be followed by the formation of tissue n, then there still remain the further differentiation stages 101, 102, 103, &c., to be provided for, which, when their time arrives, will go to form the still later tissues o, p, q, &c. Consequently the idio-plasm-B of stage 100 cannot be all consumed in making the tissue n. There must be a residual portion which will afterwards be called upon to form successively the idio-plasm-A of o, p, q, &c. Where, then, is this residual portion of idio-plasm posited? Clearly it must be posited in the nuclei of n. Thus it is that, as we began by stating, all the nuclei of any given tissue n really contain two kinds of substance,—(1) their own idio-plasm-A, which was part of idio-plasm-B of the preceding tissue, m; and (2) the idio-plasm-B, which is destined to become idio-plasms-A of succeeding tissues o, p, q, &c. Thus it follows also that the more the original idio-plasm-B is differentiated into these successive formations of idio-plasms-A the less of it remains for further differentiation, till, at the last stage of ontogeny, all the original idio-plasm-B (germ-plasm) has been thus changed into idio-plasms-A severally distinctive of all the somatic-tissues a, b, c—x, y, z,—save only the portion of it which has been carried through all these ontogenetic stages in a wholly undifferentiated condition, for the purpose of securing the phylogenetic production of the next generation. And this, of course, is secured by the portion of undifferentiated germ-plasm in question being deposited in the nuclei of germ-cells, at whatever stage of the ontogeny these may be formed.

Finally, it is evident that, at each stage of the differentiation of idio-plasm-B into idio-plasms-A, the portion concerned must be capable of self-multiplication to an almost incalculable extent,—yet this only as idio-plasm-B of the particular kind required for constructing the idio-plasm-A which is appropriate to the particular stage. Such is a necessary deduction from the terms of Weismann’s theory, inasmuch as we know that at each of the ontogenetic stages there is an incalculable multiplication of cells belonging to that stage—cells, the “cytoplasm” of which necessarily presupposes for its formation its own appropriate idio-plasm in both kinds, and this in similarly increased quantities.

From the above theory it follows that an explanation can be given of the healing of wounds (as in ourselves), of the regeneration of lost parts (as the limb of a newt), or even of the reproduction of an entire organism from a mere fragment of somatic-tissue (as in the cases already alluded to at the commencement of this chapter—viz. the leaf of Begonia, portions of sea-anemones, jelly-fish, &c.). For in all these cases of repair, regeneration, and what may be called somatic reproduction, we have only to suppose that not all the idio-plasm-B of any given ontogenetic stage is consumed in the formation of that stage, and therefore that the residue is passed on to the later stages in a latent condition. It will then be available at any time to re-develop tissue corresponding to that particular stage, should that particular tissue happen to be lost by accident or disease. For example, if some of the idio-plasm-B of the very first ontogenetic stage, or true germ-plasm, should thus be passed on in an undifferentiated condition through the somatic-tissues subsequently formed at later ontogenetic stages, then we can understand why an entire organism is reproduced from a fragment of these tissues—or of those among which particles of such residual and undifferentiated germ-plasm happen to be scattered. Similarly, if idio-plasm-B of the ontogenetic stage at which a limb is formed be not all consumed in constructing the limb, then the limb, if afterwards lost, will be reconstructed, although an entire organism will not be reproduced from a fragment of somatic-tissue. And similarly also with the mere repair of injuries, where the only overplus of idio-plasm-B is that of idio-plasm-B belonging to the very last stages of ontogeny.