6 
MR. E. RAT LANKESTER ON THE 
Plate 2. fig. 26 is important, because it shows the way in which the mouth first 
makes its appearance, and its relation to the “ rectal peduncle.” Plate 2. figs. 25 
& 26 represent the same embryo unmoved, but the focus slightly changed : hence it 
is obvious that the mouth (o) is about to eat its way into the epiblast’s wall in order to 
reach the enlarging rudimentary alimentary cavity — quite independently of the original 
point of invagination ; this is, indeed, as mentioned above, long since closed, and sub- 
sequently in its neighbourhood the rectum, at first csecal, opens to the exterior. 
Plate 2. figs. 27, 28 represent an embryo in the same stage of development, two 
views somewhat differently focused being given. They serve to confirm the disposition 
of parts ascertained from fig. 25. But in fig. 28 the focus is so arranged as to bring a 
larger number of mesoblastic corpuscles into view; in particular, above the mass of 
large cells at f, there is an indication of the strings of branched corpuscles which 
have already been seen in the phase drawn in fig. 23. 
Plate 2. fig. 29 shows the commencement of the oral invagination and the develop- 
ment of cilia on the surface round this invagination. Cilia have, however, previously 
made their appearance. 
Plate 3. fig. 30 takes a leap forward ; but the gap is to some extent filled by 
figs. 32 & 33. The individual cells are no longer represented as in the optical sections 
of previous stages, but a general superficial view of the embryo is presented. The 
surface of the embryo has now become considerably differentiated. The ciliated region 
marked f is the foot, which is now contractile and takes on special growth. The 
plications in its side (mn) indicate the commencement of the mantle-flap, whilst the 
most important differentiation of the surface is the oblong or saddle-like patch ( sh ) 
formed of large elongate epidermal cells arranged along the sides of a groove. This 
remarkable saddle-like patch is the commencement of the secreting surface which 
gives rise to the shell, or rather pair of shells. I have traced its gradual extension from 
this commencement in later stages ; but the constancy of its first appearance as a 
groove surrounded by peculiar elongate cells is the feature to which most importance 
must be attached. It will be seen from my observations on Aplysia that the first 
commencement of the shell of the larva or “ veliger form ” is there of precisely the 
same nature, viz. a groove surrounded by elongate secreting cells. In Aplysia , and also 
in Neritina , this groove is sufficiently deep to be entitled to the title of “ gland ” or 
“ follicle.” It is of the same order of structures precisely as the byssal gland, and gives 
rise to a chitinous plug in Aplysia and Neritina. In Pisidium, as will be seen from 
Plate 4. figs. 38 a, 39, the two calcareous valves do not make their earliest appearance 
in close contact one with the other. The central portion of the shell-gland is not 
concerned with them ; and since it is precisely that point which in the Gasteropods cited 
gives rise to a chitinous plug, may we not see in the ligament of the bivalve, which 
occupies so precisely the required position, the homogen of that production \ 
At present I am not prepared to go further with this subject than to suggest that 
were the open groove of the shell-area to become closed in so as to form a sac, and 
