436 ARTHUR ROBINSON. 
has already been noted in another communication (42) it 
is not necessary for me to refer to it here. 
A mere glance at figs. 15 4, 15 B, and 19 might give the im- 
pression that the celom of the rat and the mouse commences 
in the extra-embryonic area, but it must be noted after the 
fusion of the two halves of the space on the caudal side the 
allantois is projected from the posterior extremity of the 
primitive streak between the somatic and splanchnic layers 
(fig. 19, Pl. XXVII). The root of the allantois is attached to 
the splanchnic layer and somatic layers in an area which after- 
wards becomes part of the ventral wall of the enteric canal (fig. 
21, Pl. XX VII), and it becomes evident on reference to fig. 19 
that when the point X at the anterior end of the embryonic 
area and the point X’ at the posterior end are folded round 
till they meet in the middle line on the ventral surface at the 
closure of the umbilical orifice, a portion of the root of the 
allantois will be included within the body. The portion of the 
ceelom which surrounds the root of the allantois will be con- 
verted into a part of the peritoneal cavity ; therefore, as this is 
the section of the ccelom which was first formed, the coelom in 
the rat and the mouse commences in the embryonic area. In 
this respect, therefore, it differs both from the ccelom of the 
sheep and the guinea-pig. It differs also from the celom of 
the sheep in commencing as two lateral spaces instead of by an 
irregular series of unconnected cavities, and in its extension, 
which is very similar to the extension of the coelom in the 
rabbit, as described by van Beneden (8). 
In the sheep the cephalic and caudal portions of the ccelom 
apparently arise separately aud afterwards fuse together (6), 
but in the rabbit the coelom commences posteriorly, and then 
extends round the outer margins of the embryonic area as two 
lateral wings, which meet and fuse in front of the pro-amniotic 
space (8). 
In the rat and the mouse the celom commences in the 
posterior part of the embryonic area. After the fusion of its 
two primitive sections it becomes semilunar in outline, the 
convexity of the semilune passes backwards into the posterior 
