OF THE GERMINAL LAYERS IN GASTROPODA. 13 



riglit-handed spiral. Such a displacement of the cells is recog- 

 nizable when we compare Figs. 12 and 15. 



Aplysia {((.,.1, bo.i, Co.u d-^.u Figs. 48-50, PI. Ill) : — In this 

 species as well as in A. limacina the ectomere-cells of the third 

 generation are not the descendants of the first quartet of ectomeres. 

 Here it is the ectomere-cells of the second quartet, which present 

 the spindles in a left-handed spiral, the right posterior cell C2 

 taking the lead in cleavage. Fig. 48, the apical view, shows 

 the priority of cleavage in the cell Co, although sometimes we 

 meet with abnormal cases as represented in Fig. 49, in which 

 the cell «2 precedes the others in cleavage. The new daughter 

 cells are similar in size to any of the preceding generations, and 

 do not take a bright stain like those of Siphonaria. Again as 

 they are the descendants of the second generation, they stand in 

 alternation with the latter, and their positions with regard to the 

 first generation are différent from what is seen in the preceding- 

 species. 



THE FOURTH QUARTET OF ECTOMERES. 



Siphonaria («y, 4» <^'oj d-^, Figs. 15-16, PI. I) : — An hour 

 or so after the last stage, the blastomeres again exhibit the 

 spindles. This time the spindle appears first in the right posterior 

 blastomere C\ and the cleavage proceeds in a right-handed spiral. 

 The period in which this generation arises seems to differ with 

 species, as will be seen directly in the case of Aplysia. In the 

 present species its formation commences somewhat later so that 

 it appears concomitantly with that of the next generation. This 

 is easily proved by referring to Fig. 15, the apical view, and Fig. 

 16, the sagittal section seen from the left side. In the former, 



