DEVELOPMENTAL HISTOEY OF THE MOLLTJSCA. 
25 
Plate 8. fig. 21 represents a similar embryo seen from before, in such a position that 
only the ciliated border of the velum is in view. The anterior margin of the foot here 
forms the upper boundary of the figure. The want of bilateral symmetry due to the 
development of the region in connexion with the early-appearing mantle-cells (inn) is 
now apparent. The mass of tissue (int) is assuming form as alimentary canal, and is 
overlaid by the flattened out, but not disintegrated yellow yelk-spheres. In other 
specimens of this age the connexion of the intestinal rudiment with the region mn was 
obvious. The continuity was so complete as to suggest the notion of an ingrowth or 
invagination of the tissue at the point mn to form the mass int. We have, however, 
seen that it is derived from the mass jyme of Plate 7. figs. 8 & 11. In Ajylysia major 
no such structure as this int could be made out. The alimentary tract developed as a 
double gastric chamber with the yellow yelk imbedded in its walls ; no trace of intestine 
or rectal termination could be ascertained, the mass pme apparently giving rise only 
to traversing muscular bands (if to any thing). The contrast with the present case is 
very strong, and though possibly not rightly understood in the light of my present 
observations, must furnish an interesting problem affecting general principles in 
embryology. 
The series of figures in Plate 8 now must be looked at in connexion with the alimen- 
tary canal, and we can then pass through them again in connexion with other details of 
velum, foot, nerve-ganglion, &c. 
In figs. 17 & 22 certain of the cells which are to take part in the formation of the 
alimentary canal, and which have hitherto been obscured by the relatively larger deve- 
lopment of other parts, are seen to enlarge very greatly. The mass which they form is 
marked int. In fig. 22, at the point A, the cells are so arranged as to enclose a space 
as seen in the enlarged drawing (fig. 23) ; in fig. 24 a quite superficial view of the same 
group of cells is given, and fig. 25 an intermediate view. It is seen from these drawings 
that we have here large pellucid cells devoid of nucleus. 
Plate 8. fig. 26 shows that these cells have not, in the stage represented in Plate 8. 
fig. 22, attained their full growth. They are now individually of very large size, and 
occupy a great part of the embryo. This has grown considerably in size, whilst the 
relative bulk of the colourless elements of the alimentary tract and of the persisting 
yellow yelk-masses is greatly changed. In fig. 26 attention must be drawn to what is 
the most definite phenomenon to which one can point in this part of the development 
of Aplysia minor — namely, the growth of some of the colourless cells into the substance 
of the yellow yelk at the point marked in this figure int , resulting in the cutting off 
of a piece of this material from the rest of the yellow residuary yelk. This detached 
piece is marked dry. The detached piece does not retain its coarsely granular character, 
but speedily becomes broken down in substance and changed in colour to a dirty brown. 
This detached piece is rapidly invested by the colourless cells, and becomes, in the fully 
formed veliger- larva, a sac-like mass lying by the side of the anus — almost certainly the 
renal organ, the homogen of the Lamellibranch’s organ of Bojanus, and of the cuttle- 
MDCCCLXXV. E 
