DEVELOPMEN! OF GERMINAL LAYERS IN MAMMALS, 430 
pre-cephalic mesoblast, the other two are in the peripheral 
margins of the caudal portion of the paraxial mesoblast. As 
the sections approach nearer to the centre of the embryonic 
region the anterior portion of the hypoblastic ridge, from 
which the chorda and the bucco-pharyngeal membrane are 
formed, appears on the cephalic side of the sections. In 
fig. 16 D, which represents a section thirty-five micromilli- 
metres from 16 E, the celom is no longer present on the left 
side, aud the cephalic and caudal portions of the paraxial 
mesoblast are fusing. On the right side of the section the 
cephalic and caudal portions of the paraxial mesoblast are 
still separate, and each contains a small cavity in its peri- 
pheral margin. In fig. 16 C, Pl. XXV, the two portions of 
the paraxial mesoblast have just fused on the right side, 
and the ccelomic space is represented by a single small 
cavity. This section is fifty-five micromillimetres from 16 D, 
and between the two sections the coelomic spaces in the 
cephalic and caudal parts of the paraxial mesoblast unite 
together. 
During the latter part of the ninth day, as the celom is 
gradually extending in the extra-embryonic area, the allantois 
is formed. It is a solid mass of mesoblast which grows out 
from the posterior end of the primitive streak and extends 
through the ccelom towards the trophoblast, which it reaches 
and fuses with during the latter part of the eleventh or at the 
commencement of the twelfth day. It, therefore, hangs free 
within the body-cavity for a period of two days. It is at first 
a solid mass of mesoblast which, according to Selenka, is 
invaded, apparently about the eleventh day, by a diverticulum 
from the yolk-sac. I have been entirely unable to find any 
such diverticulum, and I am quite convinced that it does not 
exist. The diverticulum which Selenka figures is nothing 
more than a fold of the wall of the yolk-sac which has been 
cut in an oblique section. The hypoblast which is found 
within the placenta at a later period enters that organ from 
the margin of the yolk-sac, not by the allantois; but as this 
peculiar feature in the development of the rat and the mouse 
