OF THE GERMINAL LAYERS IN GASTROPODA. 11 



finally settled, the position they occupy is on the posterior dorsal 

 portion of the egg, instead of on the anterior dorsal as in Sl- 

 phonaria. On the other hand the cross furrow keeps its relative 

 position exactly as in the case of the preceding species (Fig. 45). 



THE SECOND QUARTET OF ECTOMERES. 



Siphonaria [a.^, bo, c^, do, Fig. 12-13, PI. I) : — After a pause 

 of about one hour, the second generation of ectomeres originates 

 again from the original blastomeres. They arise first in the 

 posterior blastomeres as in the case of the first genera- 

 tion, but this time from the right cell C. Although spindles 

 often appear in all the blastomeres at the same time as shown 

 in Fig. 13, yet the cell C is always ahead of all the others in 

 actual cleavage. The order of division here is just the reverse 

 of that in the first quartet as shown in Fig. 12, in which we 

 notice that of the second quartet cells Co, b-,, and Oo have already 

 been produced, while D is just preparing to bud off cL. These 

 ectomere-cells are of the same size as those of the first quartet, 

 and roughly speaking occupy positions alternate with these latter. 



Aplysia (a.,, bo, Co, do, Figs. 45-47, PI. III.) : — In this species 

 too, the second quartet of ectomere-cells arise from the blasto- 

 meres, the process begining as in Siphonaria from the right 

 posterior one C. Cleavage, however, proceeds in a right-handed 

 spiral, the reverse of the order in which the first generation was 

 budded off. This point as well as the relative position of the 

 newly formed generation are clearly illustrated in Figs. 45, and 

 46, which give respectively the a pical and the left lateral, views 

 of the egg. In Fig. 45 the spindles are seen in all the blasto- 

 meres except B, and that of C is most advanced. In the next 



