232 THE DEVELOPMENT OF THE CHICK 



Hatching {after von Baer). About the fourteenth day the 

 growing embryo accommodates itself to the form of the egg so 

 as to he paraUel 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. 



