THE NUCLEUS IN LATER DEVELOPMENT 425 



work within which the subsequent operations take place in a course 

 which is more or less firmly fixed in different cases. If the cyto- 

 plasmic conditions be artificially altered by isolation or other dis- 

 turbance of the blastomeres, a readjustment may take place and 

 development may be correspondingly altered. Whether such a read- 

 justment is possible depends on secondary factors — the extent of 

 the primary differentiations, the physical consistency of the egg- 

 substance, the susceptibility of the protoplasm to injury, and doubtless 

 a multitude of others. The same doubtless applies to the later stages 

 of development ; and we must here seek for some of the factors by 

 which the power of regeneration in the adult is determined and lim- 

 ited. It is, however, not improbable, as pointed out below, that in the 

 later stages differentiation may occur in the nuclear as well as in the 

 cytoplasmic substance. 



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 

 seems to leave 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. It is, however, doubtful whether this assumption is well 

 founded. 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 regener- 

 ate one another save in a few exceptional cases. In asexual repro- 

 duction, 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. 

 As indicated above, this progressive specification of the tissue-cells 



