No. 3.] POD ARK E OBSCURA VERRILL. 46 1 



on the other. In both there is found a preoral, adoral, and a 

 postoral band of cilia. In the gasteropod the apical cells give 

 rise to an apical sense organ, such as is found in many annelid 

 trochophores. The supraoesophageal ganglia and commissure 

 apparently arise from the same group of cells in annelids and 

 gasteropods. The fourth quartette in annelids and gasteropods 

 contains mesoblast in quadrant D, but is purely entoblastic in 

 quadrants A, B, and C. A fifth quartette is formed in gas- 

 teropods and some annelids (Amphitrite) and consists of ento- 

 blast only. In the gasteropod, larval mesoblast arises from the 

 same group of ectoblast cells as in Unio, differing, however, in 

 this regard, that it is found in quadrants A, B, and C, whereas 

 in Unio it is found in quadrant A only. To this list of accurate 

 resemblances in the cleavage cells may be added the fact that 

 among annelids and mollusks the axial relations of all the 

 blastomeres (except possibly the four macromeres) are the 

 same. 



"The cause of such resemblances, like the cause of determi- 

 nate cleavage and of the constancy of specific characters, must 

 be found in protoplasmic structure, and I cannot escape the 

 conviction that these likenesses belong to the same category 

 with the fundamental resemblances between gastrulae, larvae, 

 and adults. Whatever criterion of homology one may adopt,— 

 whether similarity of origin, position, history, or destiny, or all 

 of these combined, — certain of these resemblances in cleavage 

 bear all the marks of true homologies." 



The polyclade cleavage offered difficulties which were great, 

 "perhaps irreconcilable," but Conklin argues that, since we 

 find no perfect homology between adult structures, we need 

 not expect to find perfect homology between cleavage stages. 

 Professor Conklin's conclusions may, I think, be fairly given in 

 the following passage (No. 5, a, p. 198): "If organs which are 

 homologous among annelids and mollusks, such as the proto- 

 troch, the apical sense organ, the stomodaeum, and the ventral 

 plate, can be traced back in their development to certain indi- 

 vidual cells of similar origin, position, size, and history, are not 

 these cells truly homologous } If not, where in this develop- 

 mental process shall we say that homologies begin.?" 



