DEVELOPMENTAL HISTORY OF THE MOLLUSCA. 
17 
placed in the fully formed veliger larva. Plate 5. fig. 9 is in median section, whilst 
fig. 11 is somewhat more superficial. 
Plate 5. fig. 12 gives a surface-view of the same embryo, indicating the condition of 
the surface-cells at this period, as seen in the living condition. 
Plate 5. fig. 13 exhibits another embryo in the same plane of optical section 
(approximately) as that given in fig. 9. The differentiation of the outer lot of the 
original cell-mass ( x ) to form a markedly denser layer {me) is shown. In this specimen 
minute actively vibrating cilia were detected among the cells (ci). They may correspond 
to the mesoblastic cilia described in the preceding contribution in Pisidium, or may 
be only the forerunners of the general ciliation of the gastric cavity. This latter view 
is the more probable, since it is undoubtedly from cells occupying the position ci that 
the epithelium of the chief alimentary cavity must be formed in this species of Aplysia. 
At the point marked slip a thickening of the epiblast is indicated, which is the 
commencement of the secreting-area of the shell or shell-patch, as it is convenient to 
call it. In the Aplysia minor it will be seen how strongly developed this patch 
becomes, so that it readily is detached from the embryo with its delicate circular 
secretion — the rudimentary shell. It corresponds with the shell-groove of Pisidium. 
Plate 5. fig. 14. The same plane of optical section of a more advanced embryo. 
The ring of cilia (vv) which now appears, indicating the velum, is seen at the points where 
it is traversed by the plane of section. At ot the first indication of the otocysts, that 
of the right side, is seen. In Plate 5. figs. 17 & 18 the earliest commencement of 
this organ is more fully exhibited. It originates as a vacuolation of a spot in the 
epiblast near to the commencing oral invagination. It never communicates with the 
exterior ; and by the unequal development of surrounding parts it is gradually trans- 
ferred from this primitive position to that which it subsequently occupies in the foot. 
I shall speak of this again in describing the same stage in A. minor. 
In Plate 5. fig. 14 the epiblast is also seen to be considerably thickened at the 
uppermost point, v. It is here that the inward growth to form mouth and pharynx 
rapidly takes place. The history of mesoblast and hypoblast is to some extent affected 
by what is shown in the lower part of the figure. Between the darker wall-marked ime . , 
which seems to correspond with me of fig. 13, and the shell-patch there now appears a 
mass of cells ( pme ), the origin of which is quite uncertain. A similar mass appears at a 
corresponding period in Ap. minor , and they must have one of two origins ; either they 
have been “delaminated” (proliferated) from the epiblastic mass of the shell-patch, or 
they are segregated from me of fig. 13. It is really of considerable importance to deter- 
mine which view is correct ; for this mass {pme) appears to be concerned, most certainly 
in the case of A. minor , in building up the intestinal portion of the alimentary canal, 
perhaps only furnishing its outer walls. In A. major the position of this mass of 
cells does not permit one so readily to follow out its connexion with the alimentary 
canal as in A. minor. These two tracts of cell-aggregates I distinguish as inner meso- 
blast ( ime ) and parietal mesoblast {pme), without attributing definitely a particular 
MDCCCLXXV. D 
