28 



MIND IN EVOLUTION 



CHAP. 



some cases the half embryo is formed first, and the missing 

 half is regenerated from it later on. In other cases the 

 half-embryo formation is found only in the earliest stages, 

 while in others again it is entirely suppressed. Thus, to 

 illustrate the first case, Roux destroyed one of the two 

 " cleavage cells " into which the ovum of a frog divides, 

 and found that the other grew into an embryo which 



B C 



REGENERATION IN THE UNICELLULAR ANIMAL Stentor [Gruber]. 

 A. Animal divided into three pieces, each containing a fragment of the nucleus. 

 B. The three fragments shortly afterwards. C. The three fragments after 

 twenty-four hours, each regenerated to a perfect animal. 



wholly lacked one half of the body. The missing half 

 could, however, grow out of the existing half at a later 

 stage. 1 For the second case we may quote experiments on 

 the ova of sea-urchins, which divide first into two and 

 then into four cells. If at either stage these cells are 

 isolated, they continue to segment 



"as if still forming part of an entire larva, and give rise to a 



half (or quarter) blastula. The opening soon closes, however, to 



1 Verworn, General Physiology ', trans. F. S. Lee, 1899, p. 534. 



