306 



THEORIES OF INHERITANCE AND DEVELOPMENT 



E. CRITIQUE OF THE ROUX-WEISMANN THEORY 



From a logical point of view the Roux-Weismann theory is unas- 

 sailable. Its fundamental weakness is its ^//^-metaphysical char- 

 acter, which indeed almost places it outside the sphere of legitimate 

 scientific hypothesis. Not a single visible phenomenon of cell-divi- 



Fig. 132. Half and whole cleavage in the eggs of sea-urchins. 



A. Normal i6-cell stage, showing the four micromeres above (from Driesch, after Selenka). 

 B. Half i6-cell stage developed from one blastomere of the 2-cell stage after killing the other by 

 shaking (Driesch). C. Half blastula resulting, the dead blastomere at the right (Driesch). 

 D. Half-sized i6-cell stage of Toxopneustes, viewed from the micromere-pole (the eight lower cells 

 not shown). This embryo, developed from an isolated blastomere of the 2-cell stage, segmented 

 like an entire normal ovum. 



sion gives even a remote suggestion of qualitative division. All the 

 facts, on the contrary, indicate that the division of the chromatin is 

 carried out with the most exact equality. The theory of qualita- 

 tive division was suggested by a totally different order of phenom- 

 ena, and is an explanation constructed ad hoc. Roux, it is true, was 

 led to the hypothesis through an examination of mitosis ; but it is 



