18 MR. E. RAY LANKESTER. ON THE 
origin to them, or a particular further development, exceping so far as that it is obvious 
that ime forms the chief bulk of the wall of the primitive gastric sac of A. major, though 
probably not its lining epithelium. 
Plate 5. fig. 15 is another and very similar embryo, in which the same arrangement 
of parts is observed. 
Plate 5. fig. 16 is an embryo a very little further advanced and a little turned om 
its axis. The “clearing up” or “hollowing out” of the primitive gastric cavity is now 
advancing, though not yet is there any thing like a well-defined space there, but merely 
a looseness and fluidity of material, such as accompanies the formation of a cavity by 
absorption. 
Plate 5. figs. 17, 18 have been already referred to. They exhibit on a larger scale 
the earliest indication of the otocysts; fig. 17 that of the right-hand side, fig. 18 that of 
the left-hand side. This first rudiment of the otocyst developing in the epiblast may 
be termed the ‘‘ otocystic vacuole.” 
Plate 5. fig. 19 takes the development a step beyond fig. 16. ‘The alimentary cavity 
(al) is much more distinctly marked, and the mass of tissue which has grown inward 
from the epiblast to form the pharynx (ph) is in conjunction with it. The foot (f) is 
beginning to push itself forward, and the velum (v) is becoming elevated into a kind 
of cap. 
A main point of interest in this stage of development, as compared with 4. minor, 
is that the yellow yelk-granules are constituents of the mass which forms the wall of 
the primitive alimentary cavity. In A. minor they remain outside it entirely, persisting 
as the original nucleated yellow yelk-spheres, absolutely unchanged morphologically 
until the embryo is of large size and freely swimming with its alimentary canal highly 
developed ; they dwindle by absorption of their material and become relatively minute 
bodies as the embryo increases in size, but they do not, as in A. major, enter into 
the actual substance of the wall of the alimentary canal. 
A close parallel to this is seen in the development of two allied Oligochztous Anne- 
lids described by Kowatevsxy, Huaxves and Lumbricus. In the former there is a 
large quantity of nutritive matter in the form of angular granules mixed with the egg as 
laid. This granular matter, by the process of segregation and invagination (by epibolé), 
becomes confined to the central part of the embryo. The large cells of which this mass is 
formed differentiate to form the glandular lining of the alimentary canal, enclosing a 
number of the large cells as “contents” to the alimentary cavity, which are gradually 
absorbed. ‘The primitive hypoblastic wall of the alimentary cavity is thus formed 
by protoplasmic elements, each of which is distended with coarse angular granules, 
which are only gradually absorbed. This is parallel to the case of Aplysia major. 
In Lumbricus the egg is much smaller and comparatively free from an admixture of 
coarse deutoplasmic particles. The hypoblastic wall of the alimentary canal, when 
developed, is also free from them, and consists of pellucid columnar cells. This agrees 
with A. minor, excepting (and this isan important distinction, for which it is not easy to 
