DEVELOPMENT OF THE EXTERNAL FORM OPISTHOBRANCHIA. 159 



to the other authorities on this subject will be found in the literature 

 appended to this section and in the course of our account.* 



In consequence of the rich supply of yolk in the egg, gastrulation 

 seems usually to take place by epibole.-f The blastopore, at one 

 period, is a slit of variable length (e.g., in Fiona and Elysia, HADDON, 

 No. 40 ; Ercolania, TRINCHESE ; Aplysia, BLOCHMANN). This slit 

 closes from behind forward and, in some cases, altogether disappears, 

 the mouth then arising as an ectodermal invagination at the point at 

 which it closes. This is the case, according to BLOCHMANN, in 

 Aphjxia and a similar condition may, according to VOGT'S account, 

 be found in Elysia. In Fiona, according to HADDON, the slit-like 

 blastopore closes from behind forward and either passes directly into 

 the mouth or the latter is invaginated at the spot where the former 

 finally closes. Such a condition can be gathered from the descriptions 

 given by TRINCHESE (No. 125) and LANGERHANS (No. 62) of the 

 Aeolidae and Doris, but these accounts are not very clear. From 

 all these statements, however, it appears tolerably certain that the 

 mouth corresponds in position to the anterior end of the slit-like 

 blastopore. 



The changes that take place in the large entoderm-cells are 

 significant in connection with the further shaping of the embryo. 

 These have been specially observed in Aplysia by BLOCHMANN. 

 Cleavage is unequal from the first and, at the four-celled stage, two 

 of the cells are very much larger than the remaining two, and this is 

 still apparent after the abstriction of the micromeres, when we find 

 two very large and two smaller macromeres (Fig. 41 B). In conse- 

 quence of the smaller amount of the yolk contained in the latter, 

 they soon divide and give rise to a mass of small entoderm-cells, 

 while the two large macromeres (Fig. 41 / and //) are retained in 

 their full size. The ectoderm-cells grow over the entomeres and the 

 smaller entoderm-cells separate from the two large macromeres which 



* [Recent workers on this group have devoted themselves mainly to the 

 question of cell-lineage, see HEYMONS (No. XII.) and VIGUIEK (No. XXVI.). 

 MAZZARELLI (No. XVI.) has, however, made some additional observations on 

 the larval Aplysia. ED.] 



t [HEYMONS (No. XII.), who has investigated the early stage in the 

 ontogeny of Umbrella, finds that the gastrula is here intermediate between 

 the epibolic and the embolic type, as is the case in so many other Gastropods. 

 His work, which is an important one, deals largely with the cell-lineage and 

 the early ontogenetic stages. Umbrella, in its cleavage, appears to conform to 

 the normal Gastropod type, the process of entoderm-formation is quite 

 unlike that described by BLOCHMANN in Aplysia, the yolk being equally 

 distributed between the four macromeres and the entodermic epithelium 

 arising in a more normal manner. ED.] 



