THE NUCLEUS IN LATER DEVELOPMENT The foregoing conception, as• far as it goes, gives at least an intelligible view of the more general features of early development and in a measure harmonizes the apparently conflicting results of experiment on various forms. But there are a very large number of facts relating especially to the later stages of differentiation, which it leaves wholly unexplained, and which indicate that the nucleus as well as the cytoplasm may undergo progressive changes of its substance. It has been assumed by most critics of the Roux-Weismann theory that all of the nuclei of the body contain the same idioplasm, and that each therefore, in Hertwig's words, contains the germ of the whole. There are, however, a multitude of well-known facts which cannot be explained, even approximately, under this assumption. The power of a single cell to produce the entire body is in general limited to the earliest stages of cleavage, rapidly diminishes, and as a rule soon disappears entirely. When once the germ-layers have been definitely separated, they lose entirely the power to regenerate one another save in a few exceptional cases. In asexual reproduction, in the regeneration of lost parts, in the formation of morbid growths, each tissue is in general able to reproduce only a tissue of its own or a nearly related kind. Transplanted or transposed groups of cells (grafts and the like) retain more or less completely their autonomy and vary only within certain well-defined limits, despite their change of environment. All of these statements are, it is true, subject to exception ; yet the facts afford an overwhelming demonstration that differentiated cells possess a specific character, that their power of development and adaptability to changed conditions becomes in a greater or less degree limited with the progress of development. How can we explain this progressive specification of the tissue-cells and how interpret the differences in this regard between related species ? To these questions the Roux-Weismann theory gives a definite and intelligible answer ; namely, that differentiation sooner or later involves a specification of the nuclear substance which differs in degree in different cases. When we reflect on the general role of the nucleus in metabolism and its significance as the especial seat of the formative power, we may well hesitate to deny that this part of Roux's conception may be better founded than his critics have admitted. Nageli insisted that the idioplasm must undergo a progressive transformation during development, and many subsequent writers, including such acute thinkers as Boveri and Nussbaum, and many pathologists, have recognized the necessity for such an assumption. Boveri's remarkable observations on the nuclei of the primordial germ-cells in Asearis demonstrate the truth of this view in a particular case ; for here all of the somatic nuclei lose a portion of their chromatin, and only the progenitors of the germ-nuclei retain the entire aneestral heritage. Boveri
himself has in a measure pointed out the significance of his discovery. insisting that the specific development of the tissue-cells is conditioned by specific changes in the chromatin that they receive,' though he is careful not to commit himself to any definite theory. It hardly seems possible to doubt that in Ascaris the limitation of the somatic cells in respect to the power of development arises through a loss of particular portions of the chromatin. One cannot avoid the thought that further and more specific limitations in the various forms of somatic cells may arise through an analogous process, and that we have here a key to the origin of nuclear specification without recourse to the theory of qualitative division. We do not need to assume that the unused chromatin is cast out bodily ; for it may degenerate and dissolve, or may be transformed into linin-substance or into nucleoli.
This suggestion is made only as a tentative hypothesis, but the phenomena of mitosis seem well worthy of consideration from this point of view. Its application to the facts of development becomes clearer when we consider the nature of the nuclear " control " of the cell, i.e. the action of the nucleus upon the cytoplasm. Strasburger, following in a measure the lines laid down by Nageli, regards the action as essentially dynamic, i.e. as a propagation of molecular movements from nucleus to cytoplasm in a manner which might be compared to the transmission of a nervous impulse. When, however, we consider the role of the nucleus in synthetic metabolism, and the relation between this process and the morphological formative power, we must regard the question in another light ; and opinion has of late strongly tended to the conclusion that nuclear " control " can only be explained as the result of active exchanges of material between nucleus and cytoplasm. De Vries, followed by Hertwig, assumes a migration of pangens from nucleus to cytoplasm, the character of the cell being determined by the nature of the migrating pangens, and these being, as it were, selected by circumstances (position of the cell, etc.). But, as already pointed out, the pangen hypothesis should be held quite distinct from the purely physiologicalcal aspect of the question, and may be temporarily set aside ; for specific nuclear substances may pass from the nucleus into the cytoplasm in an unorganized form. Sachs, followed by Loeb, has advanced the hypothesis that the development of particular organs is determined by specific "formative substances" which incite corresponding forms of metabolic activity, growth, and differentiation.