OF THE GERMINAL LAYERS IN GASTROPODA. 27 



A simihir but Bliglitly advanced stage in Aplysia is also 

 shown in Fig. ö9. In this it is seen that unhke ^Siphonaria 

 the tii'st (ai, A,, etc.), the second {a.,, do, etc.), the fifth («ji, 

 (î»i.i, etc.), and the sixth (r/o..,, 00,2, etc.), generations of ectoderm- 

 cells cover the dorsal apex of the egg. Futhermore the 

 ectoderm- cells, as a whole, are aggregated in this species in 

 a more posterior portion of the egg than in Siphonaria. Again, 

 owing to the large size of the anterior entomeres the egg of 

 Aplysia is generally thicker in that portion than is the case in 

 the preceding species (see Fig. -54). Although not as marked 

 as in other species, the first Anlage of the trochoblast-cells can 

 be distinctly made out in the two species under consideration. 

 As will be seen clearly in Figs. 24 and 09 the apical quartet of 

 the first ectomeres is encircled by a girdle of eight cells, of 

 which four («1.1, Aj.i, etc.), i.e. the first descendants of the first 

 ectomere-quartet form the trochoblast. While the apical quartet 

 gives rise to the " arms of the cross " its further development does 

 not take place in these two species until after three germinal layers 

 are firmly established. 



In Siphoîiaria, after such a disposition of cells has been 

 completed, the cleavage of the mesomere M and of the entomeres 

 ^4, B, and G occurs synchronously (Fig. 25). Usually, however, 

 the mesomere- cell M precedes the other three, as shown in the 

 horizontal section (Fig. 25). The spindle in this cell is directed 

 almost exactly horizontally (Fig. 26) and the cleavage plane 

 thus formed conies to correspond with the median axis of the 

 egg. The resultant cells ^iii) are in their nature (piite identical with 

 those of Aplysia to which allusion has already made (p. 25). It 

 suffices here only to call attention to figures mentioned above, 

 and to Fig. 27, which shows the horizontal section through the 



