No. I.] 



THE EMBRYOLOGY OF CREPIDULA. 



121 



The cell 2a'' is the tip cell in Crepidula, and lies in the row 

 of velar cells, not posterior to it. This is not however a pro- 

 found difference, as will be shown in the section devoted to 

 the velum, since at most it means that at certain points the 

 velum in Crepidula lies one cell farther from the apical pole 

 than in Nereis. The cell 2a2, with the corresponding cells 2\i- 

 and 2c2, forms the stomodaeum in Nereis. In Crepidula a 

 very long time intervenes between the origin of the cell 2-^^ 

 and the formation of the stomodaeum, and I cannot trace the 

 derivatives of this cell into that structure. 



In general it may be said that the divisions of the cells 2a, 

 2b, and 2c, as far as they have been followed, are very much 

 the same in the annelid and gasteropod. 



In Unio Lillie ('93) has found that the cell 2ai has a peculiar 

 history : the right half, 2-d?, takes part in the formation of the 

 larval mantle ; the left half, 2ai, which in Nereis is the left 

 stomatoblast, moves into the cleavage cavity and there divides, 

 giving rise to the mesoblast which enters into the larval organs. 

 In Crepidula a similar larval mesoblast cell arises in each of 

 the quadrants A, B, and C, and probably from the cell groups 

 derived from 2a, 2b, and 2c (see p. 149)- Because of its pecu- 

 liar history the divisions of the cell 2a' in Unio are unlike 

 those of the corresponding cells in the other quadrants and 

 also unlike the divisions of 2a in any other animal hitherto 

 described. The divisions, however, of 2b and 2c are like those 

 in Neritina and the other gasteropods already described, as the 

 following table of the lineage of 2b in Unio will show : 



