30 MR. E. RAY LANKESTER ON THE 
Exeter College, and kept there under a slow-running stream of water. Only one out of 
the many ova contained in a capsule undergoes cleavage and further development. 
The others break up and furnish nutritive material to the developing individual. This 
phenomenon, which has been established in other Gasteropods, as by CLAPAREDE in 
Neritina, connects itself at once with a view which has been with much justice put 
forward by GrGENBAUR—namely, that the glands in Mollusca and Vermes which secrete 
? 
“‘deutoplasmic” material which is appropriated by the growing ovarian egg, or is 
enclosed with it in a capsule, are to be regarded as abortive portions of the ovary. 
Thus the material which feeds the favoured egg-cell, whether it be presented in the 
capsule or in the ovarian tubes, is one and the same by origin—namely, potential ova. 
The easiest way of examining the contents of the capsules of Neritina I found to be 
to open them under a dilute solution of osmic acid (‘1 per cent.). This prevented the 
breaking up of the various ova and the young embryo, which is likely to be caused by 
other media, even by iodized serum. 
In Plate 9. figs. 18, 14, 15, three stages of cleavage are represented. In fig. 13 the 
first division into two masses is commencing. ‘The separation of formative and of food- 
yelk is already quite obvious. ‘The constitution of the clear straw-tinted food-yelk, 
consisting as it does of spherical non-nucleated corpuscles, is a point of interest. When 
cleavage has advanced to a further point, they assume a more homogeneous character. 
Plate 9. figs. 16, 17 represent a polar and a lateral view of two embryos further 
advanced. They are already actually of twice the diameter of the embryos 13-15. The 
cleavage-cap of cells is gradually embracing the four spheres of residual colourless (not 
coloured as is usual) yelk. 
Plate 9. fig. 18. The enclosure is complete, and internal arrangements are in progress 
which the opacity of this species does not permit the observer to follow. The four yelk- 
spheres are still intact. 
Plate 9. fig. 19 represents the phase which has most importance for the present 
occasion. The embryo has greatly enlarged, and is assuming the well-known veliger 
form. When caught at the right angle, the shell in a rudimentary state, as a delicate 
disk, is seen to cover the thickened aboral surface. CLAPAREDE saw the shell at an early 
period, but he did not detect what is of so much interest in connexion with what I have 
described in Pisidiwm and in the deformed Aplysia minor—namely, the deep inden- 
tation in this shell-patch or shell-secreting surface occupied by the plug of chitinous 
material (pl), which in Aplysia I spoke of as the shell-plug. 
Plate 9. fig. 20 gives a more highly magnified and cleanly focused view of the same 
shell-plug and shell-patch. 
Limasx agrestis.—Plate 9. figs. 21 & 22 give two views of two different embryos of 
Limax agrestis, I kept a large number of ova of Limax and of Arion at Jena in April 
1871, and followed out the development to acertain extent. I submit on this occasion 
only the two drawings (figs. 21 & 22), because they establish the occurrence of the 
Gastrula form developed by invagination in these Pulmonate Gasteropods. 
