160 



GASTROPODA. 



are still undivided. An archenteron forms between them, lined partly 

 by the small entomeres and partly by the two persistent macromeres 



(Fig. 70). Here also there 



ed. 



ent. 



ent. 



Fig. 70. — Embryo of Aplysia limacina in optical 

 section (after Blochmann). bl, blastopore ; ect, 

 ectoderm ; ent, entoderm. 



is a suggestion of a condition 

 intermediate between an 

 epibolic and an invagination - 

 gastrula, as is said to be the 

 case in other Gastropods 

 (<■/. p. 115). The closure 

 of the blastopore and the 

 sinking in of the stomo- 

 daeum already described 

 (Fig. 71, in), take place 

 immediately after this stage. 

 Where the macromeres are not in direct contact with the ectoderm, 

 the smaller entoderm-cells spread out (Fig. 71). The gut, still partly 

 bordered by the macromeres which have shifted apart, now resembles a 

 closed sac* Up to this point there has been no sign of the mesoderm- 

 rudiment which, according to Blochmann, appears late in the form of 

 two small masses of cells lying to the right and left of the stomodaeum, 

 the origin of which could not be established. Trinchese, on the other 

 hand, described, in the Aeolidae, two large and distinct primitive 

 mesoderm-cells which may be traced back, like those of the mesoderm- 

 rudiment found by Eho in Chromodoris, to the macromeres. [See 

 the more exact work of Heymons (No. XII.) on Umbrella in this 

 connection, and footnote, p. 119]. 



At the side of the embryo opposite to the mouth, the shell-gland 

 arises as a depression which at first is shallow, but deepens later 

 (Fig. 71 A, sd) ; above this, the shell-integument is soon secreted. 

 Two cells (az), which lie on the ventral side in front of the shell- 

 gland and which, in consequence of their large size, rise above the 

 surface (Fig. 71), mark the position of the anus, which appears late. 

 These anal cells could be seen in Aplysia in earlier stages, lying at 

 the posterior edge of the blastopore. They had been already described 

 by Langerhans in several Opisthobranchs (Aeera, Aeolix, Doris) 

 and had been connected with the formation of the anus ; the same 



* [Mazzarelli (No. XVI.) does not appear to have traced the ultimate fate 

 of the two smaller macromeres, but one would imagine, from his description, 

 that they form part of the ectoderm. He regards the small entomeres seen in 

 Fig. 71 as derivatives of the two large macromeres and, judging from his Fig. 

 12, PI. x., small cells are constricted off from the macromeres. His observa- 

 tions are not clear, hut they seem to differ from those of Blochmann. — Ed.] 



