16 MR. E. RAY LANKESTER ON THE 
morphological identity between the enclosed portion of the embryo resulting from 
epibolé and the enclosed portion resulting from embolé. But, on the other hand, in 
those cases of epibolé where, as in Aplysia, and more strikingly in Neritina, there is 
clearly formative material mixed with the enclosed nutritive mass as indicated by its 
cleavage, we may look for a segregation of that formative material to form hypoblastic 
elements; and if such takes place, the enclosed mass of this case of epibolé becomes a 
true equivalent of the enclosed mass of embolé. Nevertheless it must be remembered 
that it has not been demonstrated in any one case that the hypoblast has such an origin, 
and that in the frog we have corpuscular elements resulting from segmentation, which 
serve no other purpose than that of nutritive evanescent yelk. 
The ascertainment, then, of the further arrangements and dispositions of the 
embryonic cells of Aplysia has great general interest. The difficulties of observation, 
however, entirely prevent any one set of observations from being at all conclusive as to 
these questions. 
Plate 5. fig. 9 shows an embryo in which the surface-layer of cells has condensed 
so as to form a firm “ epiblast,” consisting of but one row of cell-elements (ep). The 
yellow yelk (7y) has commenced to break up, no longer retaining its definite spherical 
form, and between the two masses of yellow granular material a mass of colourless 
closely aggregated cells has forced itself (2). This strongly contrasts with the corre- 
sponding phase in Aplysia minor, where the yelk-spheres retain their form unchanged 
(Plate 7. fig. 3). The yelk-spheres may be said in A. major to have now fused with 
the cells (2), for there is no demarcation or limit to the two masses; the individual yellow 
angular granules of the yellow yelk retain their sharp outlines, but the matrix in which 
they were imbedded seems either to have segregated and become indistinguishable from 
cells formed at the original cleavage-pole, or to have been assimilated by those cells, 
which have now worked their way between and right into the two yellow spheres. 
Plate 5. fig. 9 is a median optical section. 
Plate 5. fig. 10 gives the same embryo focused more superficially. 
Plate 5. fig. 11. There is now some differentiation in the mass of cells (x), which, 
as already explained, may contain corpuscles derived from the yellow spheres, or may 
be solely the remnant of the colourless cleavage-yelk after the separation of the 
epiblast (ep). We notice now first of all the formation of a distinct cavity (¢c), which 
must be identified with the mesoblastic cavity of Pésidiwn, and more generally of 
all the embryos of higher animals. But in addition to this the outer cells of the mass 
(x) have taken on definite character, and form a dense layer, with fine processes passing 
from them to the epiblastic wall. The comparison of this with the similar stage in 
Pisidium is instructive. 
In this and the preceding figure a pair of cells (mn) projecting from the epiblast 
are obvious. These two cells constantly appear in this stage of development in various 
Nudibranchs. They are seen when followed out to be the first commencement of the 
mantle-flap, and indicate approximately a point at which the anus subsequently is 
