14 AET. 1. — T. FUJITA : ON THE FORMATION 



all of the blastomeres except D are shown to have budded off 

 their daughter cells c^ etc, while in the latter which represents 

 a slightly earlier stage the blastomeres of which A and D are seen 

 show their spindles in an early phase. It is moreover evident in 

 both the figures, that the formation of this generation is im- 

 mediately followed by that of the fifth quartet [a^.x, etc). The 

 cells of the fourth generation are, as a rule, rather larger than 

 any of the foregoing ones. They occupy a position on the ventral 

 aspect of the egg, just under the third generation («i.i, etc). 

 By the formation of the fifth generation they, however, shift 

 their position more to the left side ; and eventually come to in- 

 terpose themselves together with the fifth quartet between the 

 cells of the second {a.2, etc) generation (Fig. 15). 



After throwing ofi* three series of ectomeres, i.e., at the forma- 

 tion of this fourth generation of ectomeres the blastomeres are 

 completely differentiated, and make no further contribution to the 

 formation of the ectomere-cells. The cleavage which occurs later 

 in the blastomeres oives rise to the Q-enerations of entomere-cells. 

 Hence, the blastomeres receive the name of entomeres with the 

 single exception of the left posterior one D. This latter ultimately 

 gives birth, beside the entomere, to the mesomere, so that it is 

 generally designated as the ento-mesoniere. 



Ajjlysia («„ h, c„ d„ Figs. 48-50, PI. Ill) :— The 

 fourth generation arises as in Siphonaria from the left posterior 

 blastomere, and proceeds in a right-handed spiral. In this 

 species too, as was mentioned in Siphonaria, the formation 

 of this generation very often takes place simultaneously with 

 that of another quartet, but here it is with the preceding (co, 

 etc), and not with the following, generation. This fact is well 

 illustrated in Figs. 48 and 50, respectively showing the apical 



