Embryogeny of the Bryozoa. 389 
Larva. 
When we open the young ovisac of any Cyclostome, we 
find it filled with small morule, of which the size varies 
from the most minute dimensions up to those of a pretty large 
and well-formed blastula. I had thought, on studying these 
little morule, that they were formed directly by means of 
a series of planes of segmentation, all passing through the 
centre of the primitive ovum, as is the case in numerous 
animals in which segmentation directly gives origin to a stage 
with a single layer of radiating cells. 
1; A thorough investigation of these youngest stages has 
shown me that this was not the case; the segmentation does 
not at all resemble the type indicated in the preceding lines. 
From the very first stages the vitelline spheres glide over 
each other, so as to form by epibolism a kind of gastrula; and 
we soon meet with stages of extremely minute size, and already 
composed of an ectodermic layer and of a free endodermic 
mass in its interior. ‘The endodermic mass becomes rapidly 
atrophied, and we get a small blastula, which does not follow 
upon a stage composed of radiating cells in which a central 
cavity is formed, but which has, on the contrary, originated 
from a true gastrula produced by epibolism in the first stages 
of the segmentation, and in which the endodermic mass has 
already disappeared. 
2. It is then that the great increase commences which 
transforms the above little blastula (psewdo-blastula) into a 
large and spacious blastula, which becomes. invaginated to 
give origin to a stage presenting a perfectly illusive re- 
sembiance to an archigastrula. The invaginated portion 
which is thus produced has nothing in common with the en- 
doderm; it represents the sac which exists in the same 
fashion asin the Escharina, and is likewise formed by invagi- 
nation. 
3. After the gastrula ( pseudo-gastrula), the ectoderm divides 
into a thick and a thin part: the former, occupying the half 
of the embryo which surrounds the external opening of the 
sac, consists of long cylindrical cells ciliated at the surface, 
and represents the oral surface; the second, occupying the 
opposite half, is formed of wider, very thin, and non-ciliated 
cells, and represents the aboral surface. 
4. To pass from this 10 the complete larva, we see the whole 
aboral surface with flat cells bury itself suddenly in the inte- 
rior of the embryo, so as to form a pallial cavity entirely 
covered by the oral surface. 
In the hatched larve the median portion of the ahoral 
