6o6 WILSON. [Vol. VIII. 



thereupon becomes active, and causes a repetition of the 

 original development. By assuming a variable latent period 

 following the stimulus, Roux is able to explain the fact that 

 regeneration takes place at different periods in different 

 animals. 



Considered as a purely formal explanation, the Roux- 

 Weismann hypothesis is perfectly logical and complete. Its 

 weakness lies in its highly artificial character ; for both of its 

 two fundamental postulates — viz : qualitative nuclear division, 

 and accessory latent idioplasm — are purely imaginary. They 

 are complicated assumptions in regard to phenomena of which 

 we are really quite ignorant, and they lie at present beyond the 

 reach not only of verification, but also of disproof. The 

 " explanation " is, therefore, unreal ; it carries no conviction, 

 and no real explanation will be possible until we possess more 

 certain knowledge regarding the seat of the idioplasm (which 

 is still an open question), and its internal composition and 

 mode of action (which is wholly unknown). In the meantime 

 we certainly are not bound to accept an artificial explanation 

 like that of Roux, however logical and complete, unless it can 

 be shown that the phenomena are not conceivable in any other 

 way. I believe, however, that they are otherwise conceivable, 

 and that by rejecting both of the Roux-Weismann postulates 

 we can give, not indeed an explanation, but a simpler and more 

 natural interpretation of the facts. 



Among recent investigators Oscar Hertwig and Driesch 

 have given the fullest and most explicit statement of such an 

 interpretation, though essentially similar views have been held 

 by a number of other writers. Hertwig (12, 14) like Kolliker 

 argues with great force that karyokinetic division is not qualita- 

 tive, but purely quantitative; that at every cell-division the 

 daughter cells, whatever their prospective character, receive 

 exactly equal kinds, as well as equal amounts, of nuclear 

 material (regarded by both Hertwig and Roux as the seat of the 

 idioplasm). With de Vries he regards differentiation as a result 

 of physiological changes in the idioplasm, subsequent to cell- 

 division, such that certain of the idioblastic units (variously 

 known as idioblasts, pangens, biophores, etc.) remain latent, 



