18 AKT. 1. T. FU.TITA : ON THE FORMATION 



THE SIXTH QUARTET OF ECTOMERES. 



Siphonaria (r/,,„ />,,,, rv„ ^Z,.,, Fig'^. 17-19, PI. T) :— Tlio 

 origin of this goiiorjitioii prosents miicli v;iri;itioii aeeording to 

 the species studied. In Siphonaria it again arises from the 

 second quartet of ectomeres. Here the cleaA^age takes place first 

 in the right jiosterior cell c. followed immediateh^ by the cell bo, 

 üo, etc,, thus taking the course of a left-handed spiral. Fig. 17 

 illustrates a horizontal section of this stage, in which all the ectomere- 

 cells of the second generation show their spindles. By the seg- 

 mentation of this stage, the ectomere-cells of the second quartet 

 are divided into two cells of unequal sizes. The newly formed 

 daughter-cell a2,2, etc. is smaller than its mother-cell ; but it comes 

 to occupy the position originall}^ occupied by the latter. As a 

 consequence, the mother-cell shifts its position ventrally toward 

 the rio'lit side, and at lensjth resis on the cells of the fourth and 

 the fifth generations. In this stage or sometimes somewhat later, 

 there takes place a noteworthy event, viz., the cleavage of the ento- 

 raesomere D (Fig. 18). The spindle lies in the low^er anterior ])art 

 of the cell so that it is easily seen in the horizontal and the 

 sagittal sections, Figs. 18 and 19. The cleavage is soon followed 

 l)y the division into two cells, the entomere-cell Di, and the 

 mesomere-cell M (Fig. 20). They present considerable ditferences 

 not merely in size, but also in general character. The entomere- 

 cell Di is of about the same size as the ectomere-cell of the third 

 quartet, and moreover like the latter has a comparatively large 

 nucleus. It is situated anteriorly, ventrally to the mesomere-cell 

 M, (Fig. 21) and maintains its direct contact with the right 

 antci'ior entomci'c />, with the cross furrow between as before 



