No. 3-] PODARKE OBSCURA VERRILL. 455 



by Wilson (No. 34, e), and which, I judge, he has subse- 

 quently abandoned, that, "We must primarily take anatomy 

 as the key to embryology and not the reverse. Comparative 

 anatomy, and not comparative embryology, is the primary 

 standard for the study of homologies." The mesoblast of 

 the mollusk or the annelid is a single continuous structure, 

 not a mixture of non-homologous structures, and the meso- 

 blast of the platode is homologous, in the proper use of the 

 word "homology," with the whole and not with a part of it. 

 Whether this conclusion would apply to more widely separated 

 groups might perhaps be doubted, for the mesoderm appar- 

 ently has very diverse modes of origin (see Montgomery, 

 No. 25), but in forms as closely related as the annelids, 

 mollusks, and platodes I believe that it is correct ; and, as 

 said before, coenogenetic modifications must have been so 

 great that we are hardly at liberty to assume that the 

 diverse modes of origin are proofs of the non-homology of 

 this layer in other cases. 



Cell and Regional Homologies. 



One of the most important problems connected with the 

 study of cell lineage, especially in its bearing on the meaning 

 of cleavage, is the question of cell homology. The wonderful 

 agreements which exist between the cleavage stages of anne- 

 lids, lamellibranchs, gasteropods, and platodes, when viewed in 

 the light of widely accepted homologies between the larvae 

 and adults of these groups, have led most workers to propose 

 for cleavage stages an homology as complete and accurate as 

 for the adult organs. And, indeed, if a blastomere of an early 

 cleavage stage in a gasteropod is identical in position and origin 

 with a blastomere in an annelid, and the descendants of each 

 give rise to an organ which can be considered homologous in 

 the two cases, the cell homology would certainly be proved. 

 As Conklin has said, " I believe there is no escape from the 

 conclusion that the protoblasts of homologous organs are as 

 certainly homologous as are the organs to which they give 

 rise ; that the protoblasts of homologous layers are as surely 



