6 ART. 1. T. FUJITA : ON THE FORMATION 



Other. When we observe a fresh specimen in such a stage, we 

 notice the existence of a period, although very short, in which 

 the two blastomeres are completely separated from each other by 

 a narrow space. In Limax, Kofoid^'^*^^ claims to have found at the 

 spot " a rather vague, transparent, and unstained protoplasmic 

 connection." In my specimens, both fresh and preserved, I could 

 not finds any instance of such connection, although Kofoid's 

 statement seems to be a highly reasonable one. The large blas- 

 tomere thus formed contributes mostly to the formation of the 

 anterior portion of the adult, while the smaller contributes to 

 the posterior. Therefore, they will hereafter be designated re- 

 spectively as the anterior and the posterior, blastomeres. The 

 eggs of various jMolluscan species which have been thus far studied 

 are not alike in the size of the two blastomeres. For example, 

 in Unio the posterior blastomere is larger, while in Neritina 

 (Blochmann)*^^ Umbrella, etc. it is smaller than the anterior one. 

 Soon, the two blastomeres again come gradually in contact 

 with each other, the surfaces by which they touch, becoming 

 flattened. Simultaneously their general configuration begins to 

 alter, and becomes somewhat elongated in the direction parallel 

 to the plane of contact. During this period of apposition the 

 anterior blastomere slightly shifts its position obliquely upwards 

 in the direction of the animal pole. In about one hour, the 

 blastomeres begin to stretch further in the direction already 

 elongated and finally divide in a plane which is at right angles 

 to the plane of contact or that of the first cleavage. This 

 phenomenon appears usually first in the anterior one (Fig. 6), 

 and is soon followed l)y the posterior complement. In the 

 blastomeres thus transformed, the furrow causing the second 

 cleavage commences almost simultaneously at the animal and the 



