6i4 WILSON. [Vol. VIII. 



it we accept the idioplasm hypothesis at all, we must accept 

 the further conclusion that the internal factors are to be 

 sought in the nature of the idioplasm. I conceive accord- 

 ingly that the first cleavage of Nereis differs from that of 

 AmpJdoxus because of an original difference of germ-plasm in 

 the two ova, and the effects of this initial difference come more 

 and more clearly into view as the cleavage proceeds. 



So it is with any ontogeny. The entire series of events 

 is primarily determined by the organization of the undivided 

 ovum that forms its first term, and, as such, conditions every 

 succeeding term. The morphological value of the individual 

 blastomere at any particular stage is the product of two 

 factors, one of which (the embryonic environment) is external, 

 while the other (the nature of the idioplasm) is internal. Only 

 under such a view can we find some understanding of the 

 remarkable fact, on the importance of which I have elsewhere 

 insisted (35, p. 441) that, in cleavage-forms that are identical 

 up to a comparatively late stage, blastomeres may exactly 

 correspond in position, mode of origin, and embryonic environ- 

 ment, and yet be of entirely different morphological value ; and 

 it is in this sense that we may regard cleavage-forms as con- 

 trolled by a definite hereditary element apart from purely 

 mechanical conditions (p. 602). 



I cannot accept Driesch's conclusion that his experiments 

 definitely overturn the principle of " organbildende Keimbe- 

 zirke"; they prove only that this principle does not extend 

 to the unsegmented ovum, or to the early stages of Echimis, 

 not that the principle is devoid of reality or value. Roux, I 

 believe, hits the mark when he says in his latest paper (pub- 

 lished after my preliminary paper on Amphioxus) : " Das Prin- 

 zip der organbildenden Keimbezirke beginnt somit erst mit 

 der Furchung eine feste Bedeutung zu erhalten ; und diese 

 seine causale und topographische Bedeutung wird mit dem 

 Fortschreiten der Furchung eine immer speziellere " (25, p. 

 310). My studies on Nereis and Amphioxus had led me inde- 

 pendently to the same conclusion (^. p. 610 ; see also No. 35), 

 though under a fundamentally different conception of the 

 nature of differentiation. To this, however, we must add the 



