THE CELL ORIGIN OF THE PROTOTROCH. 129 



regard to the origin of the prototroch, however, it must be 

 admitted that, according to von Wistinghausen's account, this 

 organ is formed from cells 

 which do not correspond with 

 any of the trochoblasts of 

 other annelids. His account 

 places us in the same predi- 

 cament with regard to the ori- 

 gin of the prototroch as with 

 regard to the origin of the 

 primary mesoblast cell. In 

 respect to both, if the account 

 is correct, Nereis Dummerillii 

 is unique. In order to bring 

 this annelid into line with the 

 others, most writers on cell- 

 lineage have expressed doubt, 



and rightly I think, aS tO the FIG. 19. ^/^/*/, 2 8-cell stage, from left side. 



accuracy of the account in 



the case of the mesoblast. The origin of the mesoblast cell, 

 however, is definitely described by von Wistinghausen, while 



he did not even pretend to 

 work out the prototroch cell 

 by cell. So we may, perhaps, 

 be permitted at least to sus- 

 pend judgment in regard to 

 the discrepancy in the origin of 

 the prototroch in this species. 

 In those annelids in which 

 the trochophore is still farther 

 suppressed, and which undergo 

 a direct development, e.g., in 

 RJiynchelmis and Clepsine, the 

 anterior quartette in the 8-cell 

 FIG. 20. scoiecoiepis, 28-ceii stage, from stage is still farther reduced. 



anterior end. . , ,. , 



Fig. 22 is from an unpublished 



drawing of Clepsine, 8-cell stage, given me by Dr. Whitman. 

 We may say in general of all the forms which we have 



