OF THE GERMINAL LAYERS IN GASTROPODA. 9 



After complete accommodation it will be seen that here again 

 the cross furrow lies between the cells B, and D, ])Oth at the 

 animal and the vegetative poles. The same phase is observable in 

 A. limacina,^-^ and also with only a slight difference in Umbrella. 

 The hypothesis advanced by Kofoid/-^^^ as to the relation existing 

 between the cross furrow and the presence of an abundant deu- 

 toplasm can not he accepted in these cases. 



THE FIRST QUARTET OF ECTOMERES. 



Sipkonaria (cci, b^, Ci, d^, Figs. 9-11, PL I) : — About two 

 hours ai'ter the complete formation of the original quartet, the 

 spindles appear simultaneously in all the blastomeres ; but actual 

 cleavage begins always at the left posterior one D. The axes of 

 the spindles in all the cells are directed obliquely upwards (Fig. 

 10), and when seen from the animal pole they seem to converge 

 towards one another (Fig. 9). The cleavage proceeds in one 

 cell after anotlier in a certain regular course. In this species 

 the course of this cleavege is in a right-handed spiral or in the 

 direction of the motion of clock-hands as shown in Fig. 9, the 

 apical view, and also in Fig. 10, the posterior side-view. In the 

 former figure we see that the blastomere D precedes all others in 

 development, presenting already its small daughter-cell. This 

 priority of cleavage and the further course of development is 

 made clear by the next figure. Here the first ectomere cell, di 

 from the blastomere D is seen to be already well formed, while 

 the formation of a^ from the blastomere A is not as much advanced. 

 In the other two blastomeres B and (7, the division is indicated 

 only by a spindle. It is stated by Kofoid, that in Limax the 



