CONNECTION BETWEEN EMBRYO AND MOTHER. 729 
—the future inner layer, endoderm or hypoblast (Fig. 398, I.). 
(2) The epiblast divides into an embryonic disc, which will 
form the epidermis, nervous 
system, etc., of the embryo, 
and an external layer, the wall 
of the embryonic sac or blasto- 
cyst, with which the disc retains 
a slight connection until the 
protective amnion is formed. 
In the outer epiblastic wall 
lacunee develop, which are 
bathed by the maternal blood, 
and the pillars of tissue 
between the lacune grow out 
into villi, which aid in this 
earliest’ connection between 
mother and offspring. Long 
before. any vascular area or 
foetal placenta is developed, 
the outer epiblastic wall has 
the above nutritive function, 
and deserves its name of 
trophoblast (Fig. 396, 77r.). 
(3) The hypoblast or inner 
mass, which is at first a solid 
aggregate of cells (Fig. 395, 
zc), becomes a sac, as a 
morula may become a blasto- 
sphere. The upper part 
Fic. 399.—Development of foetal 
membranes.—After Hertwig. 
Uppermost figure shows up-growth and 
down-growth of amnion folds. £., 
Embryo; a.f, amnion fold; @.1, amnion 
proper ; 2.2, subzonal membrane; g., the 
gut; y., umbilical vesicle or yolk-sac. 
The dotted line represents mesoderm ; 
the dark, hypoblast. The second figure 
shows origin of allantois, and the,amnion 
folds have met. The third figure shows 
increase of allantois (a/.); the dwindling 
yolk-sac (ys.); @c., amniotic cavity ; 
sz.m., subzonal membrane. The fourth 
figure shows the embryo apart from its 
membranes: #., mouth; @., anus. Note 
umbilical connection with yolk-sac. 
