OF HIE GERMINAL LAYEKS IN GASTROPODA. 25 



<liroclioii (Fig. ")()), and siibscqiiciitly divides the cell into two 

 nlinost equal halves. The dauglitev-eells (y>/, Figs. ,>7 and ;>S) ai-e 

 ovate in form and are characterized byt heir less granular contents. 

 Their position is in a strict bilateral symmetry with respect to 

 each other, and the plane in which they come in contact almost 

 coincides with the future median axis of the body. 



Previous to this stao'e the nuclei of the entomere-cells A, 

 and 7> lie usually near the posterior end in the middle line 

 uf each cell as is shown in Fig. 5ö. They, however, have 

 commenced to shift their position toward the right (Fig. ''){]). 

 This goes so far in A that the nucleus reaches the ventral side 

 of the anterior end as will l)ecome intelligible l)y a reference 

 to Figs. ÖG and Ö8. It would seem that such a shifting of the 

 nucleus in the two entomeres could have no other purpose than 

 to prepare for the formation of the future entoderm-cell. This 

 latter event arises shortly after the bilateral symmetry becomes 

 well pronounced by the clejp'age of the mesomere-cell M. The 

 formation of the entoderm-cells has its begining in the posterior 

 cells ; and indeed in the right component cell C (Fig. ")?). In 

 the formation of this generation the sj^indle lies almost horizon- 

 tally, and the daughter-cells detached which are known as the 

 entoderm-cells (A^, B^, ({, Figs. o9 and GO) are rather small 

 and situated alternately with, and outside of their mother-cells 

 {A, II 0). 



THE EIGHTH QUARTET. 



Siphonaria (ry, ., />,.,>, ^,.,, ^/,.,, Figs. 22-24, PI. IT) :— With 

 the difterentiation of the entomere-cell D^ the behaviour of 

 individual cells becomes hardlv ascertainable in a surface view. 



