730 MAMMALIA. 
of this sac forms the lining of the incipient gut, while the 
lower portion, following the contour of the blastocyst wall, 
forms the lining of the umbilical vesicle (cf. the Chick). 
From this vesicle or yolk-sac the embryo becomes folded 
off, and the connection between the two is narrowed, just 
as in the chick, into a canal—the vitelline duct, which is 
part of the “umbilical cord,” entering the embryo at the 
future navel. (4) Between the epiblast and the hypoblast 
of the embryo, the 
mesoblast develops, 
splitting into an 
outer, parietal, or 
somatic, and an 
inner, visceral, or 
splanchnic layer. 
The cavity between 
these is the incipient 
body cavity. A 
double fold of so- 
matic mesoblast, 
carrying with it a 
j single sheet of epi- 
== blast, rises up round 
Fic. 400.—Diagram of fcetal memlnanes about the embryo, 
—AfterTumer. arching over it to 
“the dark i mésdblasts OP), uaiheat verte a, form the amnion, 
nes ® Te ee aeiors Over the embryo 
Batty? a, allantois : ah ‘nay be here faben ie the folds of amnion 
Hypize does Wot chew that tae amelon fie conse, Meee A, Cupola, 
of hoth epiblast and mesoblast. and the inner layers 
: of the double fold 
unite to form the “amnion proper,” while the outer 1ayers 
also unite to form a layer lying internally to the epiblastic 
blastocyst wall,—and termed by Sir William Turner she 
subzonal membrane. The folds of amnion are continued, as 
the diagram shows, ventrally as well as dorsally, so that the 
subzonal membrane surrounds the embryo beneath the 
blastocyst wall, while a splanchnic layer of mesoblast grows 
round about the hypoblastic yolk-sac. The space between 
the two layers of mesoblast is continuous with the body 
cavity of the embryo. The epiblastic outer wall or tropho- 
a 
cc~e 
‘t 
a 
