THE NUCLEUS IN LATER DEVELOPMENT 321 



G. THE NUCLEUS IN LATER DEVELOPMENT 



The foregoing conception, as far as it goes, gives at least an in- 

 telligible view of the more general features of early development and 

 in a measure harmonizes the apparently conflicting results of experi- 

 ment 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 sub- 

 stance. 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 tJie nuclear substance wJiicJi 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 trans- 

 formation 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 re- 

 markable observations on the nuclei of the primordial germ-cells in 



