I 



The Development of Ischnochiton. 609 



Annelids. Among the Annelids this quartette appears to be sub- 

 ject to considerable variation as regards the size of cells, and the 

 accounts of their various fates differ considerably. In Nereis, ac- 

 cording to Wilson, a^-^, b^-^ and c^-^ are destined to enter the 

 stomodaeum, which observation in general meets with approval, but 

 regarding the destiny of the parent cells a^-^ etc. differences of 

 opinion arise. According to ^'ilson the descendants of these cells 

 divide more or less vertically, giving rise to "a series of large polygonal 

 cells lying below the prototroch" which he terms post-trochal cells. 

 None of these are said to enter the prototroch as secondary trocho- 

 blasts but from the anterior group with the accompanying descendants 

 oi a^ and h^ (third quartette) "arises the ectoblast surrounding the 

 stomodaeum and forming the superficial part of the body-wall of the 

 antero-lateral region"; and regarding the fate of these cells in the 

 right and left quadrants he says "the postero-lateral region on each 

 side (between the margin of the ventral plate and the prototroch) is 

 occupied by the descendants of the post-trochal cells" (offspring of 

 a 2-1 on the left side, and of c^-^ on the right) and of c^ and d^. 

 These various statements have been called in question by both 

 LiLLiE and Mead, and the objections are strengthened by the history 

 of these cells in Ischnochiton. Mead finds in both Amphitrite and 

 Clymenella that the second quartette in the three quadrants furnish 

 secondary trochoblasts and the same thing occurs in Ischnochiton and 

 possibly in Crepidula. Also Lillie and Mead criticise Wilson's 

 statement that a^-^ and c^-^ aid in the formation of the latero- 

 dorsal region. In Unio the second stomatoblast occupies this place 

 and in Amphitrite since the descendants oi 2d form almost the entire 

 trunk such a fate for these cells is out of the question. In Ischno- 

 chiton this region is formed by the products oî 2d and also by the 

 dorsal borders of 3 c and 3d. 



There is thus a wide difference of opinion regarding the fate of 

 these cells, but since there is such an essential agreement in this 

 regard between the development of Unio, Amphitrite and Ischnochiton 

 it seems probable that Nereis will be found to offer no exception when 

 restudied; and it also seems probable that in the majority of An- 

 nelids and Molluscs the second quartette will furnish secondary trocho- 

 blasts and will extend from the prototroch to the blastopore even 

 though the development of the first somatoblast introduces a great 

 distortion in the original radial symmetry. 



