224 THE DEVELOPMENT OF THE CHICK 
(3) The Albumen-sac. The allantois in the course of its 
expansion over the embryo, between amnion and chorion, reaches 
the sero-amniotic connection; it must then either divide and 
grow round on each side of the connection, or evaginate the 
chorion above the connection and carry it as an overlapping 
fold on beyond. The latter is what actually happens, and there 
is established as a consequence an overlapping fold of the chorion 
containing an extension of the allantois (Fig. 128); the space 
beneath this fold terminates, naturally, at the sero-amniotic 
connection. In the meantime the cleavage of the mesoblast 
has separated chorion and yolk-sac as far as the neighborhood 
of the yolk-sac umbilicus, where the viscid albumen has accumu- 
lated. The latter is situated not opposite to the yolk-stalk, 
but near the posterior pole of the yolk-sac, with reference to 
the embryo, 7.e., usually towards the narrow end of the shell. 
Now the allantois growing around the yolk-sac from all sides 
reaches the neighborhood of the albumen and enters an evagina- 
tion of the chorion that wraps itself around the albumen, thus 
initiating the formation of a double sac of the chorion enfolding 
the albumen and containing between its two layers an extension 
of the allantois. The latter is therefore separated everywhere 
from the albumen by the thickness of the chorion. The superior 
fold of the albumen-sac is the same fold that overgrows the 
sero-amniotic connection, and the albumen-sac is therefore pro- 
longed beneath this fold to the sero-amniotic connection itself, 
which, as we have seen, becomes perforated, thus admitting 
albumen into the amniotic cavity. 
The ectoderm lining the albumen-sac is two-layered, and the 
cells next the albumen tend to be cubical or swollen, and fre- 
quently vesicular, owing apparently to absorption of albumen. 
In the neighborhood of the yolk-sac umbilicus, papilla-like pro- 
jections of the ectoderm into the albumen are common (Fig. 129). 
But these do not occur over the remainder of the albumen-sac 
of the chick, as described by Duval for the linnet; nor do they 
possess a mesodermal core. 
Prior to the union of the mesoderm over the yolk-sac umbili- 
cus, the yolk forms a hernia-like protrusion into the albumen- 
sac (sac of the yolk-sac umbilicus, see Fig. 129), which is, however, 
retracted as the mesoderm ring closes over the yolk-sac umbilicus. 
The vitelline membrane ruptures at an early period of the incu- 
