No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 28 







t"'', Fig. 114) are even more diminutive than the primary, and 

 diminutive trochoblasts are to be expected in suppressed 

 trochophores, on the principle of the homology of cleavage 

 cells. 



Wilson ascribes a very different fate to these three cells in 

 Nereis. They arise in the same way as in AmpJiitrite and 

 Clymenella, and undergo one division in the same manner (the 

 next divisions are not recorded) ; but two of the cells instead 

 of contributing to the prototroch form the " latero-dorsal region 

 of the trunk " (l.d., text Fig. IV). 



I think, however, that the fate of these cells in Nereis requires 

 further confirmation, for the following reasons : (i) in Aniphitrite 

 and Clymenella such a fate of ct'^ and d"^ is out of the ques- 

 tion, because all but a very minute portion of these cells enters 

 the prototroch, and because the latero-dorsal region, in Amphi- 

 trite at least, is formed from another cell, the somatoblast d^; 

 (2) in Nereis only one division of the cells is recorded, while 

 the origin of the rest of the prototroch remains unsolved ; (3) 

 such a fate of these cells involves a serious discrepancy between 

 the behavior of the somatic plate (ventral plate) cells in Nereis 

 and in the other forms, while in Amphitrite and Clymenella — the 

 only forms in which their fate has been accurately ascertained — 

 there is a remarkable agreement. 



The stoniatob lasts {Nereis) and larval mesoblast (Unid). — In 

 Nereis Wilson records the peculiar fate of the sister cells of the 

 secondary trochoblasts, i.e., of the cells cC""", If"'^, c^'^. They form 

 a ring about the stomodseum and hence are called stomatoblasts. 

 But Lillie shows that in Unio a^'^ has an equally peculiar fate : 

 it forms the larval mesoblast. The inference has been drawn 

 and was explicitly stated by Lillie that in this case cells of the 

 same origin have different destinies, — a blow to cell homology 

 (Lillie,^ p. 37). Stated in this way, that a°'^ is a stomatoblast 

 in Nereis and the larval mesoblast in Unio, the inference is well 

 warranted. But, if we examine more critically the fate of these 

 cells in both forms, an obvious fallacy appears. Wilson does 

 not describe in detail the cleavage of the cell in question, 

 though he shows that it divides at least once in the same direc- 

 tion as in Amphitrite and Clymenella. Only one product of 



