CRITIQUE OF THE ROUX-WEISMANN THEORY 



409 



the missing half by a peculiar process, related to regeneration, which 

 Roux designated as post-generation. Later studies showed that an 

 isolated blastomere is able to give rise to a complete embryo in many 

 other animals, sometimes developing in its earlier stages as though 



Fig. 185. — Dwarf and double embryos of Amphioxus. 

 A. Isolated blastomere of the two-cell stage segmenting like an entire egg {cf. Fig. 183, D). 

 B. Twin gastrulas from a single egg. C. Double cleavage resulting from the partial separation, 

 by shaking, of the blastomeres of the tw^o-cell stage. D.E.F. Double gastrulas arising from such 

 torms as the last. 



Still forming part of a complete embryo ("partial development"), 

 but in other cases developing directly into a complete dwarf embrj^o, 

 as if it were an egg of diminished size. In 1891 Driesch was able 

 to follow out the development of isolated blastomeres of sea-urchin 



