No. I.] THE EMBRYOLOGY OF CREPIDULA. lOI 



ula which corresponds to the division of the terminals in 

 Nereis, by which the middle cells are formed. 



If now we attempt to compare the cells of the molluscan 

 cross with the corresponding cells in Nereis, we find that the 

 basal cells correspond to the intermediate girdle cells (la'-^— id'-^) 

 in Nereis, while the terminal cells are represented by cells 

 which lie outside the prototroch, and, except in the case of the 

 posterior arm, are secondarily separated from the intermediate 

 girdle cells by the products of the trochoblasts, which shove 

 in between the terminals and the intermediate girdle cells. 

 About the time that these cells, 2a'-'-2d'-', are formed in 

 Nereis the intermediate girdle cells divide into an inner and 

 an outer part which correspond to the basal and middle cells in 

 the cross of Crepidula. There are thus formed four radiating 

 rows of cells which run out to the prototroch, and correspond, 

 at least in origin, to the arms of the molluscan cross. In the 

 case of the posterior row there is a plain cell series, with basal, 

 middle, and terminal cells exactly as in Crepidula icf. Diagram 

 13, a, also Wilson's Fig. 38). The terminal cell (x^) in this 

 case is a product of the first somatoblast and is not separated 

 from the middle cell by the prototroch, as is the case in each 

 of the other cell series. The following table indicates what 

 cells in Nereis correspond to the cross cells in Crepidula : 



Crepidula. Nereis. 



Apicals Rosette 



Basals Intermediate Girdle, Inner 



Middles Intermediate Girdle, Outer 



Terminals Post-trochals and x^ 



Every cell of the annelidan cross can be identified with the 

 molluscan, and conversely every cell of the molluscan cross can 

 be identified with that of the annelid. Yet on the other hand, 

 with the exception of the apicals, not a single cell of the anne- 

 lidan cross is found in the molluscan, nor a single cell of the 

 molluscan cross in that of the annelid. In fact, the arms of 

 the molluscan cross lie midway between the arms of the anne- 

 lidan cross and vice versa. The fact is, there are two distinct 

 systems of radiating cell series at the animal pole in both anne- 

 lids and mollusks, the one lying in the median and transverse 



