No. 3.] PODARKE OBSCURA VERRILL. 457 



them are as follows : " The general later history of the blasto- 

 meres thus formed is as follows : In the polyclade the first 

 group of micromeres gives rise to the entire ectoblast, the 

 second and third groups to the mesoblast, the macromeres to 

 the entoblast. In the mollusk and annelid, on the other hand, 

 the second and third groups of micromeres give rise to ecto- 

 blast, like the first set, and the mesoblast arises subsequently. 

 This remarkable divergence between the polyclade on the one 

 hand and the mollusk and annelid on the other is a fact of 

 capital importance, for it proves that cells having precisely the 

 same origin in the cleavage, occupying the same position in 

 the embryo, and placed under the same mechanical condi- 

 tions, may nevertheless differ fundamentally in morphological 

 significance." 



Extending the comparison in the mollusks, Wilson describes 

 the peculiar cross figured by Blochmann in Neritina, and com- 

 pares it with the similar structure found in Nereis. Making 

 all due allowances for Blochmann's error, which Conklin had 

 corrected, he finds that the two structures are composed of 

 cells of a very different generation, and occupy a very differ- 

 ent position with reference to the axis of the body, in the 

 annelid, from what they do in the mollusk. Wilson's conclu- 

 sion is that these structures are analogous and not homologous, 

 and their origin is in some way connected with mechanical 

 conditions of cleavage. 



Lastly, Wilson shows that the velum of the mollusk and the 

 prototroch of the annelid have a very different cell origin, but 

 is careful to insist that this fact does not, in his mind, invali- 

 date the idea of an homology between the completed organs. 



Wilson decides, at the end of his discussion, that it is neces- 

 sary to be very cautious about drawing morphological conclu- 

 sions from the comparative study of early cleavage stages. 

 Blastomeres having precisely the same mode of origin and 

 precisely the same spatial relations to the rest of the embryo 

 are by no means necessarily equivalent, either physiologically 

 or morphologically, and the early cleavage stages in themselves 

 have little morphological value. The respective value of the 

 blastomeres must be determined by their ultimate fate, and 



