196 CONKLIN. [Vol. XIII. 



cleavage he points out some fundamental differences between 

 these early stages in the annelids, gasteropods, and polyclades, 

 and concludes (p. 455): "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." 



Lillie ('95) has taken much more positive ground for the 

 homology of blastomeres among annelids and mollusks, and he 

 was justified in so doing because of the truly wonderful resem- 

 blances which he was able to demonstrate between the lamelli- 

 branch and the annelid. 



I have attempted to show that the differences of cleavage 

 between the annelids and the gasteropods, upon which Wilson 

 lays emphasis, are only apparent and not real, and that there- 

 fore we cannot deny the general homology of blastomeres 

 among annelids, gasteropods, and lamellibranchs. 



Concerning the polyclade cleavage I can offer nothing new. 

 The differences here are very great, perhaps irreconcilable, and 

 certainly this is true of other types of cleavage, such as the 

 bilateral, the centrolecithal, and the meroblastic. But to affirm 

 the homology of blastomeres within certain groups is not to 

 assert that they are everywhere homologous, nor that they are 

 completely homologous. The mesoderm of the adult mollusk 

 differs very considerably from that of the annelid, the trunk 

 region in the two groups is widely different, and we need not 

 expect to find the protoblasts of these structures completely 

 homologous. 



The fact is there are no perfect homologies between adult 

 annelids and mollusks, and therefore we need not expect to 

 find peffect homologies between their larvae, germ layers, or 

 cleavage stages ; but, since final homologies are invariably based 

 upon earlier ones, we should expect to find that blastomeres in 

 general show resemblances and differences corresponding to the 

 resemblances and differences of the end stages, and this is just 

 what we find in the cases mentioned. 



An incidental result of these observations is to bring the 

 annelids and mollusks more closely together than has heretofore 



