DEVELOPMENTAL HISTOEY OE THE MOLLUSCA. 
15 
yelk, and consequently the yellow mass or residual yelk shares in the first cleavage. 
In both species of Aplysia it is only this one cleavage which the coloured or residual 
yelk undergoes. In other Gasteropods, e. g. Neritina, it cleaves a second time, so as to 
form four masses; whilst in other cases, as also in the Batrachia among Vertebrata, we 
know that yelk which corresponds to what is here called “ residual” (that is, yelk which 
does not itself build up structure) may exhibit a very extensive cleavage, and the 
corpuscular or cell-elements therefrom resulting be nevertheless gradually broken down 
and absorbed. 
At the lower pole of the egg (Plate 5. fig. 1) a shrunken vesicle, marked B,, is 
seen escaping from the colourless yelk. It appears to be the remains of the germinal 
vesicle, and has been frequently observed by others in a variety of mollusks, being 
sometimes spoken of as the “ Richtungsblaschen.” Plate 5. figs. 2, 3 represent the results 
of the first two cleavage-furrows. The yellow yelk is in the condition of two larger 
balls, the white yelk in the condition of two smaller balls. 
Plate 5. fig. 4. The yellow yelk divides no further; but the white yelk now presents 
four masses instead of two. 
Plate 5. figs. 5, 6, 7. These continue to multiply and spread over the two balls 
of yellow yelk, which they finally enclose. Clear pellucid nuclei of large size occur in 
the yellow spheres of A. minor at this period (compare the figures, Plate 7), but, 
curiously enough, are altogether absent here. 
Plate. 5. fig. 8 shows some of the klastoplasts or cleavage-products of the white yelk 
after their complete investment of the two spheres of residual yelk. These cleavage- 
products not only invest the yellow masses, but are piled up at one pole, the original 
cleavage-pole. I sought here for some indication of the 6r«s£rwZa-invagination ; but 
obtained no evidence of it. In a recent paper, Dr. Emil Selenea has contrasted the 
process of invagination as “ embole,” with that of overgrowth (such as occurs here and 
in Loligo ) as “ epibole.” It is not yet clear how far they are equivalent processes or 
reciprocally exclusive * The presence of a large mass of “ deutoplasm ” or food-yelk is 
what, more than any thing else, seems to necessitate epibole ; and we require much more 
numerous and detailed accounts than we at present possess of the origin of the hypo- 
blast in various animals before asserting that the enclosure of the mass of residual yelk 
(containing often or invariably some formative as well as nutritive material) by the 
marginal increase of the cap of small cleavage-products is essentially the same thing 
as the enclosure of the hypoblast by invagination. If it were so we should certainly 
have, in cases of epibole, to look for the exact equivalents of the invag’inated hypoblastic 
corpuscles in corpuscles arising from or making themselves apparent in the mass of 
residual or coloured yelk. In cases where this enclosed residual yelk does not give rise 
to the hypoblast (the chick, osseous fish, Loligo 1), but in which the latter is derived by 
a process of “ lamination ” from the enclosing mass of cleavage-cells, there can be no 
* March 7th, 1875. — At the present moment I incline altogether to the view sustained by Kowalevsky in 
his invaluable researches on Euaxes and Lumiricus, to the effect that these two processes are one and the same. 
