232 THE DEVELOPMENT OF THE CHICK 
Hatching (ajter von Baer). About the fourteenth day the 
erowing embryo accommodates itself to the form of the egg so 
as to lie parallel to the long axis with its head usually towards 
the broad end near to the air-chamber. Sometimes, however, 
the embryo is turned in the reverse position (von Baer). The 
head is bent towards the breast, and is usually tucked under 
the right wing. Important changes preparatory to hatching 
take place on the seventeenth to the nineteenth days. The 
fluid decreases in the amnion. The neck acquires a double bend 
so that the head is turned forward, and, in consequence, the beak 
is towards that part of the membranes next to the air-chamber. 
The intestine is retracted completely into the body-cavity, and 
on the nineteenth day the yolk-sac begins to enter the body- 
cavity. On the twentieth day the yolk-sac is completely included, 
and practically all the amniotic fluid has disappeared. The 
chick now occupies practically all the space within the egg, 
outside of the air-chamber. The umbilicus is closing over. The 
ductus arteriosi begin to contract, so that more blood flows 
through the lungs. The external wall of the allantois fused with 
the chorion still remains very vascular. 
Now, if the chick raises its head, the beak readily pierces 
the membranes and enters the air-chamber. It then begins to 
breath slowly the contained air; the chick may be heard, in some 
cases, to peep within the shell two days before hatching, a sure 
sign that breathing has begun. But the circulation in the allan- 
tois is still maintained and it still preserves its respiratory func- 
tion. When the chick makes the first small opening in the shell, 
which usually takes place on the twentieth day, it begins to 
breathe normally, and then the allantois begins to dry up and 
the circulation in it rapidly ceases. It then becomes separated 
from the umbilicus, and the remainder of the act of hatching is 
completed, usually on the twenty-first day. 
