422 ARTHUR ROBINSON. 
phery of the mass where they are in contact, if not in direct 
continuity, with the flat cells which form the greater part of 
the wall of the vesicle. 
It is stated that the flat cells on the outer surface of the 
inner mass either disappear entirely, or they fuse with the 
cells immediately beneath them to form the epiblast of the 
germ; and in either case, after the flat cells over the outer 
surface of the inner mass have disappeared, it is just as pos- 
sible that the remainder of the vesicle wall hangs in con- 
tinuity with the peripheral flattened cells derived from the 
inner layer of the inner mass—that is, with the hypoblast, as 
with those cells which constitute the epiblast; but in the case 
of the rabbit there is no evidence which will completely sub- 
stantiate a statement that either the one or the other of these 
possibilities occurs. It is certain that the portion of the 
vesicle wall, which is at first formed by a single layer of flat- 
tened cells, eventually becomes didermic, and that the change 
from the single to the double-layered condition commences in 
the vicinity of the inner mass, whence it gradually extends to 
the opposite pole of the ovum. It is stated that the didermic 
condition is produced by the extension of the hypoblast round 
the inner surface of the primitive wall, but no satisfactory 
proof has been brought forward in support of this statement. 
The cells of the extending layer are from the first flattened, 
like those over which they are extending, and nuclear division 
has not been clearly demonstrated either in the inner or the 
outer layer. 
It appears to me, therefore, that the evidence which has 
been obtained from the study of the rabbit’s ovum does not 
conclusively substantiate the statement that the outer wall of 
the primitive blastocyst is epiblastic in nature, and that the 
hypoblast extends round its inner surface. 
If the evidence in support of the latter view is incomplete in 
the case of the rabbit, it is still more so in the case of the bat ; 
for although (fig. 6, Pl. XXIII) van Beneden and Julin (4) 
figure the greater part of the blastocyst wall as composed of a 
single layer of flattened cells, there is no definite proof forth- 
