584 DEVELOPMENT OF VESSELS AND DIGESTIVE CAVITY. 
first formed by a layer of cells; and no muscular structure is 
seen in it, until after its regular pulsations have commenced. 
_It is in these vessels that the first blood is formed; and the 
same process appears to be continued through the whole period 
of incubation, the yolk being progressively converted into 
blood, and this blood being 
conveyed .by the great 
trunks which collect it into 
the body of the embryo. 
Looking at the yolk-bag 
in the light of a temporary 
stomach, its vessels may 
be likened to those which 
take so large a share in the 
act of absorption from the 
digestive cavity of the 
adult (§ 218). 
759. During the same 
early period of incubation, 
the layers of the germinal 
membrane begin to exhibit 
various folds, which after- 
wards serve for the forma- 
tion of the several cavities 
Fig. 319.—Empryo or Brrv, witx tae of the body. The points of 
lary inte oe. — vascular area, after four it which lie b eyon d the 
extremities, and which 
spread-out from the sides of the embryo, are doubled-in so 
as to make a depression upon the yolk ; and their folded edges 
gradually approach one an- 
other under the abdomen, 
which lies next the in- 
terior of the egg. In this 
manner is formed the per- 
manent digestive cavity ; 
sles ge FG Sl el va ae which is “first a combs 
e, embryo; f, g, layers of germinal mem- pouch communicating with 
brane; , heart; s, stomach. the yolk-ba g, b ya wide 
opening, as seen at s, fig. 320 ; but which is gradually separated 
from it by the narrowing of this orifice (fig. 322), the connecting 
portion being elongated into a duct (fig. 321, 6). Thus we may 
