16 ART. ]. — T. FUJITA I ON THÉ FORMATION 



is much further to the right. Moreover, it is to be observed that 

 the difference in the amount of cytoplasm contained in the 

 anterior and the posterior halves of the egg causes the gradual 

 displacement of the ectomeres in general toward the posterior end. 

 This seems to be due partly to the fact that during all the cleavage 

 processes the axes of the spindles in the anterior blastomeres always 

 have a tendency to be inclined toward the posterior end, so that 

 an ectomere-cell budded off generally lies dorsally and posteriorly 

 to its mother cell. The posterior position of ectomeres is marked 

 even from the stage of the first ectomere generation ; thus in 

 Fig, 45 which shows the first quartet of ectomeres in their 

 proper position we see how they lie more on the posterior blasto- 

 meres. The same arrangement is also continued up to quite late 

 stages. Although this tendency is noticeable in other yolk-laden 

 Molluscan eggs it does not seem as pronouned as in Aplysia, for 

 their blastomeres shows no such marked difference in nature as 

 there is in this species. 



THE FIFTH QUARTET OF ECTOMERES. 



Slphonaria («2.1, /^.u g-ia, fh.u Figs. 15-16, PL I) : — As 

 I hiive already stated the formation of this generation takes place 

 in conjunction with the preceding one. It arises from the second 

 quartet of ectomeres. As is seen in Fig. 15 the cleavage makes 

 its first start from the right posterior cell Co and its course is in 

 a right-handed spiral. The daughter cells Co.u etc thus formed 

 are as large as those of the fourth generation. They are situated 

 more veiitrally than the mother cell and finally interpose them- 

 selves between the cells of tiie preceding generation, pushing the 

 latter more to the left along the right spiral. From this time 



