No. I.] 



THE EMBRYOLOGY OF CREPIDULA. 



117 



these (2b^-^, Fig. 56) should be the right lower cell shown in 

 Fig. 47. Still the whole identification of the cells of this region 

 must be considered as more or less doubtful. It is, however, 



much more certain that the large cells 2b'-^-^'-' and 2b'-^-^-'-^ 

 of Figs. 62, 63, 69, 70, 71 are the same cells in each of these 

 cases, and that they are the protoblasts of the velum in this 

 region of the embryo.^ 



Comparisons. 



Blochmann ('81) has described the divisions of the cells of 

 this quartette in Neritina up to the stage when there are four 

 cells in each of the quadrants, but while his figures agree per- 

 fectly with Heymons' ('93) work on Umbrella and my own on 

 Crepidula, his interpretation of the figures is at variance with 

 the results of all recent studies on the cell lineage of gastero- 

 pods. Reason has already been given (p. 65) for believing 

 that Blochmann was mistaken in his derivation of some of 

 these cells ; and a correction of his interpretation has been 

 suggested which, while finding confirmation in his figures, 

 would bring his account into agreement with the work, particu- 

 larly, of Heymons, Kofoid, and myself. 



1 See Note p. 204. 



