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



(lorsiil and the ventral views, and it has also the same length on 

 both snrfaces. 



Aplysia : — The egg of this species when freshly deposited is 

 at almost the same stage as that of Sijihonaria. It has not yet 

 discharged the polar bodies. The process of fertilization, however, 

 shows no material difference from that of the preceding species. 

 It is worthy of note that the polar bodies appear in the small 

 cytoplasmic portion of the egg. After the polar bodies are com- 

 pletely formed the egg begins gradually to elongate and at last 

 assumes an ovoid shape, the pointed end being toward the 

 cytoplasmic portion. The first cleavage plane appears just as 

 in Siiihonaria from both the animal and the vegetative poles 

 (Fig. 39). The result of this division is the formation of two 

 blastomeres totally different in size as well as in quality (Fig. 

 40). The larger blastomere is heavily laden with deutoplasm ; 

 while the smaller one is almost wholly devoid of it. Such a 

 striking difference in the two blastomeres is observed in only a 

 few cases in Gastropoda even in the so called yolk-laden eggs, 

 of Umbrella, Planorbis, etc. Of these two blastomeres the larger 

 half represents the future anterior portion of the body, and the 

 smaller half its posterior portion, these corresponding to the 

 smaller and the larger blastomeres of Siphonaria. The further 

 changes accompanying the first cleavage, such as the gradual and 

 mutual accommodation of the blastomeres take the same course 

 as in Siphonaria already fully described above. The second 

 cleavage in this species takes place first in the posterior half (Fig. 

 41, PL II). The new cells thus generated are, as in the case 

 of Siphonaria, designated as A, B, 0, and I) (Fig. 42). Their 

 i-elation to the future axis of the body coincides with that of the 

 former species and therefore requires no further explanation. 



