2(\ ART. 1. — T. FUJITA : ON THE FOE^FATION 



Tlio formation of this generation was therefore made ont in 

 Siphonaria from sections. In this species the first ectomere 

 generation gives birth to the eighth quartet of ectomeres (Fig. 22), 

 the riglit posterior cell r, taking precedence over tlie others. The 

 spindles lie in rather a horizontal plane (Fig. 23), and the 

 course of the division is in a left-handed spindle. The newly 

 formed cells r^i 9, bi.o, etc., which are shown in the next Fig. 24, 

 are much larger than their mother-cells «1, b^, etc., with which 

 they lie on the same level and the same radii. After this cleavage 

 the first ectomere generation takes the shape of a rosette, the cross 

 fin-row l)eing almost obliterated. At this stage the normal and 

 typical form of the egg is rather oval in a horizontal view, nar- 

 rowing toward the posterior side (Fig. 27). In a side view, how- 

 ever, it takes a wedge shape with the thicker end turned also 

 toward the posterior side. About this time the ectomere-cells 

 lose their spherical contour, and losing their individuality become 

 more or less flattened, altogether presenting the appearance of a 

 layer (Fig. 20). Hence they will hereafter l)e called the ectoderm- 

 layer. The ectoderm is, moreover, disposed more or less in dif- 

 ferent zones. Fig. 24 illustrates the egg in this stage, which is 

 seen from the apical pole showing clearly the exact positions as 

 well as the mutual relations of the cells. The highest or dorsal 

 zone is occupied l>y the first {a^, h^, etc.), the third («u, /;, 1, etc.), 

 and the eighth (r/,,., /-»i.o, etc.) generations of ectoderm-cells, and the 

 ventral zone (Fig. 25) is formed of the fourth (o^o, b., etc.), and the 

 seventh (r^a.i.i, b.,_^.u etc.), generations of ectoderm-cells and the 

 entomere-cells {A, B, C, D^), together with the bilateral cells (re 

 and Ic) and the mesomere-cell M ; while the lateral zone is 

 ])ordcred with the ectoderm-cells of the second (a.,, h.,, etc.), the 

 fifth (^2.1» Ku etc.), and the sixth (r^,„, /;.,.,, etc.), generations. 



