4oS 



JXIIEKITAXCE AXD DEVELOPMENT 



The hypothesis mainly rests upon a quite different order of phenom- 

 ena, namely, on facts indicating that isolated blastomeres, or other 

 cells, have a certain power of self-determination, or "self-differentia- 

 tion" (Roux), peculiar to themselves, and which is assumed to be pri- 

 marily due to the specific quality of the nuclei. This assumption, 

 which may or may not be true,^ is itself based upon the further assump- 

 tion of qualitative nuclear division of which we actually know nothing 

 whatever. The fundamental hypothesis is thus of purely a priori 

 character; and every fact opposed to it has been met by subsidi- 



A B 



Fig. 184. — Normal and dwarf gastrulas oi Ampkioxus. 



A. Normal gastrula. B. Half-sized dwarf, from an isolated blastomere of the two-cell stage. 

 C. Quarter-sized dwarf, from an isolated blastomere of the four-cell stage. 



ary hypotheses, which, Hke their principal, relate to matters beyond 



the reach of observation. 



^ Such an hypothesis cannot be actually overturned by a direct 



I appeal to fact. We can, however, make an indirect appeal, the 



I results of which show that the hypothesis of qualitative division is 



j not only so improbable as to lose all semblance of reality, but is in 



fact quite superfluous. It is rather remarkable that Roux himself 



led the way in this direction. In the course of his observations on 



the development of a half-embryo from one of the blastomeres of 



the tw^o-cell stage of the frog's egg, he determined the significant 



fact that the half-embryo in the end restores more or less completely 



1 Cf. p. 426. 



