OF THE GERMINAL LAYERS IN GASTROPODA. 15 



view, and the sagittal section seen from the right side. The 

 ectoniere-cells of this generation are a little larger than any of 

 the preceding. When fully adjusted (Fig. 51), they take their 

 positions, as in the first species interposing themselves between 

 the cells of the second {a-i, etc) and the third («2.1, etc) quartets. 

 In this species also, this generation is the final ectomere product 

 from the blastomeres. Henceforth, the blastomeres are called 

 entomeres or ento-mesomere according to their characters just as 

 in Siphonaria. 



At about this time a noticeable change occurs with regard 

 to the relative positions of the anterior and the posterior ento- 

 meres. By the repeated cleavage already gone through, it is seen 

 that the posterior half is gradually brought upwards toward the 

 apical pole. This shifting increases with development and is 

 combined with a slight movement toward the right side. If we 

 refer to Fig. 44 a right lateral view of the blastomere stage, and 

 to Figs. 46 and 49, representing the posterior views of the dif- 

 ferent stages, one before the formation of the second generation, 

 and the other after the formation of the third generation, we 

 can easily comprehend the above mentioned change in the cell 

 arrangement. At the outset both the anterior and the posterior, 

 halves of the egg stand ahnost on the same level (Fig. 44). By 

 two consecutive cleavages of the first and the second quartets of 

 ectomeres, tlie posterior half has shifted its position almost half 

 way up the anterior components (Fig. 46). Finally the former 

 seems to stand high up, and to surmount, the latter. That there 

 is at the same time a slight shifting toward the right is seen by 

 comparing Figs. 46 and 49. In the former the plane of contact 

 of A and B is seen through the right portion of D and near the 

 contact plane of D and Ö. In Fig. 49 the latter contact plane 



