THE VITAL IMPETUS 131 



Every time the embryo divides this process is repeated 

 Thus each of the (theoretically) 1028 cells of the 

 blastula contains xoWth of the substance of each 

 chromatic granule in the fertilised ovum. 



Pfiuger and Roux (in 1883 and 1888 respectively) 

 were the pioneers in the experimental study of the 

 development of the ovum, and the results of their 

 work and that of their successors has, more than any- 

 thing else in biology, modified and shaped our notions 

 of the activities of the organism. Roux found or 

 thought so at least, that the first division of the frog's 

 egg marked out the right and left halves of the body, 

 the one blastomere giving rise to the right half the 

 other to the left half. The next division, which 

 separates each of these blastomeres, marked out the 

 anterior and posterior parts of the embryo. Thus :— 



Anteriorf--^ ,^^ Anferior 



I^rt [^ -^l Rlahf 



Foster: oi\. yPosferior 



Fig. 14.— The frog's egg in the 4-blastomere 

 stage seen from the top. 



Now in an experiment which has become classical 

 Roux succeeded in killing one of the blastomeres in 

 the 2-cell stage, while the other remained aUve. 

 The uninjured blastomere then continued to develop, 

 hut it gave rise to a half-embryo only. 



Upon these experiments the Roux-Weismann 

 hypothesis of development— the " Mosaik-Theorie "— 

 was developed. The lay reader will see how obviously 

 the facts of nuclear division and the experimental 

 results indicated above lend themselves to a mechanistic 



