REGENERATION IN EGG AND EMBRYO 



217 



across the middle line, but whether the two phenomena are causally 

 connected, or are merely due to a coincidence, could only be determined 

 by further experiment. The observations themselves are not beyond 

 question, for the two planes do not always coincide, and may be even 



FlG. 61. After Roux. A. Section of semi-blastula of frog's egg. B. Half-embryo. C, Cross- 

 section of last (reversed right and left in B and C). D. Anterior half-embryo. 



ninety degrees apart. These cases of divergence were thought by 

 Roux to be due to an unobserved shifting of the developing embryo, 

 but it is improbable that all cases can be accounted for in this way. 



Roux carried out his experiment by plunging a hot needle into 

 one of the first two blastomeres, so that it is injured to such an extent 



