OF THE GERMINAL LAYERS IN GASTROPODA. 19 



(Fig. 20). The inesomere-cell, uii the other hand, is large, and 

 transkicent, and now conies to occupy a position almost in the 

 median axis of the egg. 



Aplysia {((■>.■>, b^., c.>.->, d.,;,, Figs. 03-55, PI. Ill) : — Yolk-laden 

 eggs of Mollusca present nuicli difference in the origin of the 

 present (piartet of ectomeres. In Unibrella it arises from the third 

 generation and in Neritina from the fifth. In the species of 

 Aplysia other than the one studied in the present investigation, 

 this generation is described as arising from the fourth quartet of 

 ectomeres. In the present species, however, it is the outcome of 

 the second generation just as in Siphonaria ; and the course of 

 cleavage is likewise quite identical in both cases. The spindle in 

 the ectomere-cells of the second quartet lies almost horizontal, so 

 that the daughter-cells «...j, etc. stand at the same level with 

 their mother-cells ; and when fully accomodated they push the 

 eetomere-cells of the fifth generation to the left, finally interpos- 

 ing themselves between these latter and the mother-cells. Fig. 

 53, the optical horizontal section from the animal pole shows 

 that the right posterior cell c-i of the second generation of ecto- 

 mere-cells has already propagated its new daughter-cell t'2.2 ; 

 while its left component f/^ still presents a spindle. From this the 

 course of cleavage and the relative position of this new series of 

 daughter-cells will be easily comprehended. In this figure it will 

 also be seen at a glance that the formation of the ectomere genera- 

 tions no longer follows with a strict regularity as in earlier stages, 

 and spindles are present simultaneously in cells other than the 

 second quartet of ectomeres. Such an irregularity in cleavage is 

 also seen in Fig. 54, the optical sagittal section near the median 

 line, the division of two consecutive sets of ectomeres here arising 

 almost at the same time. 



