No. I.] THE EMBRYOLOGY OF CREPIDULA. 99 



although the two crosses have exactly the same structure, they 

 have a completely different origin." From this statement I 

 should be compelled to dissent, for I do not believe that they 

 are alike even in structure. The cross in Nereis is purely 

 radial in position, dexiotropic in the gasteropods. It ultimately 

 lies in the median and transverse planes in the moUusk, mid- 

 way between these planes in the annelid. It is formed almost 

 entirely from the apical cells in the mollusk, from the ter- 

 minal cells in Nereis, and corresponding with this difference 

 the terminal cells are much the largest ones in the cross in 

 Nereis, while they are the smallest ones in the gasteropods. 

 And again, the posterior arm in Nereis is like each of the 

 others, whereas in Crepidula and Neritina it becomes very dif- 

 ferent in structure. The cells composing the cross in Nereis 

 and Crepidula are shown in the following scheme : 



The cross forms relatively much later in Nereis than in Crepid- 

 ula, as is shown by the exponents used in designating the cells. 



In Nereis the arms of the cross are formed entirely from the 

 cells la'-'-^— id'-'-^, which in Crepidula I have called the periph- 

 eral rosette. These cells are formed at exactly the same 

 division of the apical quartette in Crepidula and Nereis, viz., 

 the third ; in exactly the same direction, viz., slightly laeotropic, 

 almost radial (Figs. 44 et seq., and Wilson's Fig. 27) ; and they 

 lie in exactly the same position, viz., between the turret cells 

 (Wilson's trochoblasts) peripherally and the apical cells cen- 

 trally. In Crepidula the two anterior peripheral rosette cells are 

 secondarily separated from the turret cells by the lateral exten- 

 sion and consequent junction of the arms of the cross ; the two 

 posterior peripheral rosette cells remain in contact with the 

 turret cells. Figs. 49 et seq. In both Crepidula and Nereis the 

 peripheral rosette cells, la'-'-^-id'-'-^, divide in nearly the same 

 direction (radial in Nereis; slightly bilateral, almost radial, in 

 Crepidula), forming a cell series in each quadrant which radiates 

 from the apex. Figs. 51, 53, 62, and Diagram 13. These radiat- 



