4o8 



INHERITANCE AND 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), pecuKar 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 quahtative 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 Amphioxus. 



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

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



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

 the reach of observation. 



Such an hypothesis cannot be actually overturned by a direct 

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

 results of which show that the hypothesis of qualitative division is 

 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 two-cell stage of the frog's ^gg, he determined the significant 

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



1 Cf. p. 426. 



