426 INHERITANCE AND DEVELOPMENT 



is no doubt due in part to differentiation of the cytoplasm. There is, 

 however, reason to suspect that, beyond this, differentiation may sootier 

 or later involve a specification of the nuclear substance. When we 

 reflect on the general role of the nucleus in metabolism and its signifi- 

 cance as the especial seat of the formative power, we may well hesi- 

 tate 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 Ascaris 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-neclei retain the 

 entire ancestral heritage. Boveri himself has in a measure pointed out 

 the significance of his discovery, insisting that the specific develop- 

 ment of the tissue-cells is conditioned by specific changes in the 

 chromatin that they receive,^ though he is careful not to commit him- 

 self 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 zvithout recourse to the theory of qualita- 

 tive division. We do not need to assume that the unused chromatin 

 is cast out bodily ; for it m.ay 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 appUcation 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 metaboHsm, and the 

 relation between this process and that of morphological synthesis, 

 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, 



1 ' 



9^, P- 433- 



