330 PROCEEDINGS OF THE ACADEMY OF [Jul}', 



divides and at this stage the cross consists of sixteen cells. Still later 

 the apical cells bud off small cells between the bases of the arms, the 

 ''rosette" cells of Nereis and Crepidula (fig. 12). In every one of 

 these details of cleavage the history of the cross in Fulgur is like that 

 in Crepidula. The latest stage in which I have Ijeen able to identify 

 all the cells of the cross is one slightly later than the one shown in 

 fig. 12. At this stage the cross consists of the following cells: 



Apicals 4 



Rosettes 4 



Basals 7 



Middles 7 



Terminals 4 



Total 26 



In later stages the entire cross cannot be recognized ^^•ith certainty. 

 In figs. 13-16 the center of the cross, where the apical cells meet, can 

 be located, and the terminal cells may be seen more or less clearly, but 

 the other cells cannot always be surely identified. In fig. 13 the arms 

 of the cross can be plainly seen, each consisting of four cells in a single 

 linear series, as well as the four derivatives of the turret cells which 

 lie between the arms of the cross, but the apical and rosette cells 

 cannot be identified. In fig. 14 the cross is scarcely apparent at all, 

 and no part of it can be identified with certainty. In fig. 15 a cross 

 is shown with four cells in a series in each arm, just as in fig. 13. Fig. 

 16 shows a central plate of cells surrounded by heavy lines w^hich 

 probably represents the cross; the center of the apical cells may be 

 recognized, more or less doubtfully, at the place marked bj^ the cross 

 line, and the terminal cells are indicated by the stipples. Midway 

 between the terminal cells on each side of the plate is a group of four 

 cells, also enclosed in heavy lines, Avhich are the derivatives of the 

 turret cells. There are thirty-six cells in the cross in this latest stage 

 in which it is recognizable, eight of which are derived from the second 

 quartet. Deducting these second quartet cells and including the 

 turret cells, we find that there are forty-four cells of the first quartet 

 shown in pi. XXV, fig. 16. 



In Crepidida seven large cells of the anterior arm of the cross form 

 a peculiar "apical cell plate," while a number of large ciliated cells 

 derived from the posterior arm and the posterior turret cells form a 

 large "posterior cell plate," which becomes the head vesicle of the 

 larva. In Fulgur none of these structures are visible at any time, and 

 although the cross may be recognized clearly enough in its earlier 



