No. I.] 



THE EMBRYOLOGY OF CREPIDULA. 



87 



Diagram 8. — Crepidula : 92 ectoblast cells 

 (109-cell stage). Shading and heavy lines 

 as in preceding diagrams. The egg is rep- 

 resented as if all the ectoblast cells could 

 be seen from the apical pole, though actually 

 many of the peripheral cells lie far down on 

 the sides, or even on the ventral face of the 



rior, the right, and the left, continue to divide in the same way 



and at nearly the same time, 



though the cells of the anterior 



arm become larger than those 



of the others, and this entire 



arm becomes broader, though 



scarcely as long as either of 



the others. 



After the stage shown in 

 Diagram 8 the right and left 

 middle cells in each of the three 

 arms just mentioned divide in a 

 purely bilateral manner. These 

 cleavages are not only sym- 

 metrical with reference to the 

 median plane of the embryo, 

 but the time at which the cells 

 divide shows that the cleavage 

 is bilateral and not radial, the middle cell on the posterior side 

 of the transverse arms dividing before any of the others, Fig. 

 49. The right middle cell in each case divides in a dexiotropic 

 direction, the left middle cell in a laeotropic, Figs. 49, 50, and 

 Diagram 9. In this way a right and left intermediate cell, 

 ja^i.2.2.1.1 g^nd la'-^-^-^-', etc., is formed in each of these three arms. 



Fig. 50, which is a view from the apical pole, shows the 

 cross after the intermediate cells have been formed. This 

 figure and the next one. Fig. 51, are particularly interesting, 

 since they show a polar body attached at the point where the 

 four apical cells come together. This is the last stage in 

 which I have found the polar bodies attached to the ^g^^ 

 though they are found still later free in the egg capsules and 

 sometimes within the alimentary canal of the embryos. 



Very soon after the formation of the intermediate cells the 

 peripheral rosette cells divide almost in a radial direction into 

 central and peripheral portions, la'-'-^' and ia''-^% etc.. Fig. 51, 

 which are almost equal in size. The division is purely bilateral, 

 and the two posterior cells divide a little before the two anterior 

 ones, as was the case with the middle cells. 



