12 ART. 1. T. FUJITA : ON THE FOEMATION 



figure also, the order of cleavage is clearly indicated inasmuch as 

 the blastomere C has already almost detached its new daughter 

 cell Co ; and D is about to divide. In this species the second 

 quartet of ectomere-cells are much larger than the first set and 

 occupy positions alternate with these as in Sijihonaria. 



THE THIRD QUARTET OF ECTOMERES. 



Siphonaria («1.1, ^1.1, c'l.i, dx,i, Figs. 14-15, PL I) : — In 

 a majority of the Molluscan eggs direct segmentation from the 

 blastomeres stops temporarily with the second quartet of ectomeres, 

 the third being derived mostly from the first quartet of ectomere- 

 cells. This is also the case in the present species. In my pre- 

 liminary note on the development of the present species com- 

 municated to the Zoological Magazine already referred to, I noted 

 that this generation also originates from the blastomeres. A 

 renewed investigation has shown that this view was a mistaken 

 one, having been caused by the abnormal condition of the speci- 

 mens then accessible. In reality, this new generation arises from 

 the first quartet of ectomere-cells. Cleavage begins from the first 

 descendant {d-^ of the left posterior blastomere and proceeds in 

 a right-handed spiral. In Fig. 14, the apical view of this stage, 

 di.i alone has been budded off; while all the others show the 

 spindle in a more or less similar state. These ectomere-cells of 

 the third generation or trochoblast cells, as they are usually 

 called, are smaller than any of the preceding generations. 

 Nevertheless, they are rather conspicuous, coloring deeply when 

 stained. They lie in the same level and in alternation with the 

 ectomere-cells of the first (piartet, ])ushuig the cells of the second 

 quartet {a->, h etc) to the left, in the order which indicates a 



