No. I.] THE EMBRYOLOGY OF CREPIDULA. 67 



It will be seen at once that both Blochmann and Kofoid fail to 

 identify the cells of the third quartette with the corners of the 

 ectoblastic plate, and consequently mislabel the whole of the belt, 

 carrying the proper designations one cell too far to the right 

 in the one case, and one cell too far to the left in the other. 

 This proposed correction of Blochmann's account and of 

 Kofoid's modification is further supported by a figure of the 

 Q^g of Neritina of this same stage, given by Biitschli ('77), PI. 

 XVII, Fig. 3a, in which the position of the cells plainly shows 

 that the angles of the ectoblastic plate are formed by the third 

 quartette, while the two cells on each side between the angles 

 have evidently come by division from a single cell. 



With this slight modification of Blochmann's account Neri- 

 tina is made to agree in the matter of the belt cells with Nereis, 

 Umbrella, Limax, Unio, four species of Crepidula, Urosalpinx, 

 Fulgur, Sycotypus, and lUyonassa, and at the same time Bloch- 

 mann's statement as to the derivation of the " Urvelarzellen " 

 is confirmed, and Kofoid's contention for the alternation of 

 cleavages is satisfied. 



III. The Segregation of the Mesoblast and Entoblast. 



I. Fonnation of the Mesentoblast. Figs. 21, 22, 

 Diagram 4 {p. 60). 



At the stage just described, with twenty micromeres and 

 four macromeres, the left posterior macromere divides in a 

 laeotropic direction, as shown in Fig. 21. The cell thus 

 formed is very much larger than any of the micromeres, and, 

 unlike them, contains a considerable quantity of yolk. This 

 cell, although formed by a laeotropic division, remains in nearly 

 the same position in which it was first separated from the 

 macromere until a much later stage. Fig. 33. Like the belt 

 cells it is partly overlapped by the micromeres which lie nearer 

 the apical pole, but a considerable part of it is exposed on the 

 surface. In a strict use of the term, therefore, it cannot be 

 said at this stage to form the middle layer any more than the 

 belt cells form a middle layer. In fact, it is neither a "layer" 

 nor is it " middle," and yet from a part of this cell most of the 



