230 THE DEVELOPMENT OF THE CHICK 
epithelium over the entire extent of the septa and forms loops 
along the free border. The arteries do not communicate directly 
with this network according to Popoff, and the course of the 
circulation from arteries to veins is not clearly described by this 
author. 
The allantois fuses with the yolk-sac in the region of the 
yolk-sac umbilicus, and anastomoses arise between the veins of 
the allantois and those of the yolk-sac. 
Ultimate Fate of the Yolk-sac. On the nineteenth day of 
incubation, the yolk-sae slips into the body-cavity through the 
umbilicus; which thereupon closes. The mechanism of this 
process is of considerable interest. The yolk-sac is still a volu- 
minous organ, and equal to about one sixth the weight of the 
embryo. It is therefore inconceivable that it could be “drawn 
into” the body-cavity by means of its stalk, which has only the 
intestine for attachment. The process is much more complex 
and may be briefly described as follows: We have already seen 
that the inner wall of the allantois fuses with the amnion on the 
one hand; distally it is connected with the yolk-sac. Now this 
wall of the allantois is muscular, and it is probable that its con- 
traction is the first act in the inclusion of the yolk-sac within the 
body-wall. It is aided in this, however, by the inner wall of 
the amnion, 7.e., that part of the amnion arising from the umbili- 
cus and not fused with the allantois. This part of the amnion 
surrounds the yolk-stalk, and is itself richly provided with muscle 
cells, forming a crossing and interlacing system. It is carried 
down and over the volk-sac to about its equator by the allantois, 
and when the yolk-sac is half taken into the body-cavity, it reaches 
its distal pole and fuses there. Now if the egg be opened at 
this stage in the process and this wall of the amnion cut through, 
it contracts rapidly to a fraction of its former area (Virchow). 
It is apparent, then, that the tension of this membrane on the 
yolk-sac must exert a continuous pressure that tends to force it 
into the body-eavity. It is in this way, then, by contraction 
of the inner walls of the allantois and of the amnion, that the 
yolk-sac is pressed into the body-cavity. 
The umbilicus is therefore closed by the mere act of inclusion 
of the yolk-sac, for the inner amniotic wall is attached on the 
one hand to the body-wall, and on the other to the distal pole 
of the yolk-sac. A minute opening is left in the center of the 
