300 OUTLINES OF CHORDATE DEVELOPMENT 



changes preparatory to hatching take place on the seven- 

 teenth to 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 toward that part 

 of the membranes next to the air-chamber. The intestine is 

 retracted completely into the body-cavity, and on the nine- 

 teenth 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 breathe slowly the contained air; the chick may be heard, 

 in some cases, to peep within the shell two days before hatch- 

 ing, a sure sign that breathing has begun. But the circulation in 

 the allantois is still maintained and it still preserves its respi- 

 ratory function. 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." 



References to the literature are given at the end of Chapter V. 



j 



