DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS, 411 
of the cephalic projection, the posterior end of the second 
portion is carried upwards, that is, away from the yolk-sac, 
and forward. Thus the second portion is first horizontal (fig. 
18 A), but as its posterior extremity rises it becomes: oblique, 
then vertical, and at last again oblique, but with the ex- 
tremity which was posterior now anterior (fig. 19 4) ; and the 
whole of the mesial portion of this area forms the anterior 
wall of the fore-gut—that is, it becomes the bucco-pharyngeal 
membrane. 
Whilst this movement of the second portion is taking place 
the first portion retains its original position (fig. 19 A, 
Pl. XXVII), and the cavity which, in the latter part of the 
ninth day, was only found laterally in the mesoblast of this 
area, now extends across the middle line. 
During the course of the tenth day the anterior end of this 
portion of the embryonic area is folded backwards until it 
forms the anterior boundary of the umbilical orifice, and its 
two surfaces are reversed in position. In this manner the 
ventral wall of the fore-gut is completed. The cavity in the 
mesoblast of the ventral wall of the fore-gut is evidently the 
mesial portion of the pericardial cavity ; it extends from the 
bneco-pharyngeal membrane to the anterior boundary of the 
umbilical orifice. 
The hypoblast of the bucco-pharyngeal membrane is not 
distinguishable, in mesial longitudinal sections, from the re- 
mainder of the hypoblastic ridge, which is seen to become 
gradually thinner as it is traced backwards along the dorsal 
wall of the fore-gut. 
In transverse sections of embryos in the early part of the 
tenth day the characters of the hypoblastic ridge behind the 
bucco-pharyngeal membrane are very similar to those de- 
scribed at the end of the ninth day, but it is, in relation to the 
embryo, proportionally longer; and it is separated from the 
vitelline cavity for a somewhat greater distance by the in- 
growth of a layer of flat cells beneath it, and the characteristic 
features of its posterior extremity are better marked. 
Its posterior part, for a distance of 70 uw, has the form of a 
