CRITIQUE OF THE ROUX-WEISMANN THEORY 



307 



safe to say that he would never have maintained in the same breath 

 that mitosis is expressly designed for quantitative and also for qual- 

 itative division, had he fixed his attention on the actual phenomena 

 of mitosis alone. The hypothesis is in fact as complete an a priori 

 assumption as any that the history of scholasticism can show, and 

 every fact opposed to it has been met by equally baseless subsidiary 

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

 reach of observation. 



Such an hypothesis cannot be actually overturned by an appeal to 

 fact. When, however, we make such an appeal, the improbability of 



A B 



Fig. 133. — Normal and dwarf gastrulas of Amphioxus. 

 A. Normal gastrula. B. Half-sized dwarf, from an isolated blastomere of the 2-cell stage. 

 C. Quarter-sized dwarf, from an isolated blastomere of the 4-cell stage. 



the hypothesis becomes so great that it loses all semblance of reality. 

 It is rather remarkable that Roux himself led the way in this direc- 

 tion. In the course of his observations on the development of a half- 

 embryo from one of the blastomeres of the two-cell stage he determined 

 the significant fact that the half-embryo afterivards regenerated the 

 missing Iialf, and gave rise to a complete embryo. Essentiall)' the 

 same result was reached by later observers, both in the frog (Endres, 

 Walter, Morgan) and in a number of other animals, with the impor- 

 tant addition that the half-formation is sometimes characteristic of 

 only the earliest stages and may be entirely suppressed. In 1891 

 Driesch was able to follow out the development of isolated blasto- 



