10 ART. 1. — T. FUJITA : ON THE FORMATION 



spindles gradually shift their positions during this cleavage. Such 

 does not seem to be the case in Sii^lionaria : the axis of the spindle, 

 as may be partly gathered from the figures cited above, remains 

 unchanged in direction throughout the whole of this phase. The 

 newly formed cells, however, moves lightly when they come to adjust 

 themselves to one anotlier. Four daughter cells a■^, bi, Ci and di 

 (Fig. 11) thus propagated form the first quartet of ectomere 

 cells, lie, when completely detached, on the dorsal side of the 

 mother-blastomeres. They are small in size and are at first 

 merely an aggregation of spherical bodies, which however soon 

 begin to be applied one against another. When finally settled, 

 they seem to have been shoved somewhat towards the anterior 

 end (Fig. 11), Now in this new quartet the cross furrow is 

 again formed by a juxtaposition of the ectomere-cells bi and c/j 

 just as it was in the preceding stage by their mother-blastomeres 

 B, and D (Fig. 11). Thus the furrow of the ectomere-cells and 

 that of the blastomeres are formed by the cells genetically related 

 to one another. The lines of these two furrows form a small 

 angle with each other, i.e., neither runs parallel with, nor stands 

 at right angles to, the other as is the case in many other species. 

 Aplysia (a^, h^, c^, d^, Figs. 43-44, PL III) : — The form 

 tion of the first quartet of ectomeres here also commences 

 first in the posterior blastomere (Fig. 44), most usually from its 

 left component D, and is followed immediately by the cells C, B, 

 and A in the order named (Fig. 43). Thus the cleavage proceeds 

 in a left-handed spiral, i. e. in the direction just the reverse of 

 that of /Siphonaria. This direction is contrary to that ascertained 

 by Blochmann in A. limaoina, and also in other forms of yolk- 

 laden egg noted by various authors. The first quartet of ectomeres 

 formed are much smaller than the mother blastomeres ; and when 



