22 MR. FE. RAY LANKESTER ON THE 
which the potential foot is to the left. In this figure a darker mass of cells (x) is distin- 
guished from a paler group (pme). There can be no doubt that the cleavage-corpuscles 
enclosed by the epiblast are now in process of arranging themselves, to lay the founda- 
tions of particular groups of organs; but whether # in this case is to be regarded merely 
as a mass of primitive cells from which pme have become detached, or as a group 
destined to give rise to the hypoblast, subsequent phases of development do not enable 
me to decide. Jam inclined to take the latter view, especially on looking at the position 
occupied by the mass 2 in fig. 5. 
Plate 7. fig. 5 is a little further advanced than fig. 4. Already the circle of cilia 
belonging to the velum are present, and the first invagination of the epiblast for the 
pharynx (ph). The mass @ is now clearly separate from pme; and it is now time to 
point out that the subsequent development of the embryo most fully agrees with the 
view that this mass (2) coming into close relation with the yellow spheres, gives rise 
to the hypoblast of the alimentary canal, whilst the mass pme, which is still in part 
actually continuous with the epiblastic wall (see fig. 11), forms the muscular wall of the 
alimentary canal, and especially develops the whole of its terminal part, being gradually 
eaten into by the cavity of the alimentary canal by the growth of the hypoblast: that 
is to say, in other words, the cavity of the main chamber or stomach is first formed by 
the development of the mass a, whilst pme forms the outer wall of the intestine, into 
which an outgrowth from the stomach gradually extends. Ido not wish to attach any 
importance to these statements beyond that of suggestions; for the investigation is a 
very difficult one on account of the smallness of the embryos and their want of clearness 
in detail of structure, though in this respect they are better than most molluscan embryos. 
Plate 7. fig. 8 shows the pharynx further advanced (ph); the shell already exists as 
a delicate pellicle (sh), and the foot (f) is beginning to push. Now is the earliest period 
at which I have seen the otocyst (ot) in this species. On account of the position in which 
the embryo is lying on the glass slip, the otocyst is not brought to the edge of the section, 
but is seen lying in the foot. It is, however, still near the surface, and is in the 
condition of a vacuole excavated in the thick epiblast of this part. 
Plate 7. fig. 6 gives a much more superficial view of an embryo of the same stage 
in a reversed and oblique position. The otocyst (ot) is seen near the surface in the 
foot-region. But the most important feature in this drawing is the shell-patch and 
shell-groove (shgr), which are seen here quite superficially. The close similarity of 
this structure to the shell-groove of Pisidium cannot be overlooked. ‘This is the earliest 
stage also at which the pigment-spots (one on each side) (pg) are visible. They are 
small superficial vesicles, at first circular in outline, containing four finely coloured pink 
granules. ‘They enlarge and become oval, whilst the number of granules which they 
contain increases. I shall only speak of them as pigment-spots, for their function is 
altogether obscure. 
Plate 7. figs. 7, 9, 11 represent embryos of one and the same age, not quite so far 
advanced as that of fig. 8. Their positions (accidentally assumed as they lay on the 
