IX.] HATCHING. 303 



There is thus a completely double circulation formed, 

 in which all the blood of the left ventricle is arterial, 

 and all the blood of the right ventricle venous, and 

 there is at no part of the circulation a mixture of venous 

 and arterial blood. 



As early as the sixth day movements, as we have 

 said, may be seen in the limbs of the embryo upon 

 opening the egg. We may conclude that after this 

 epoch spontaneous movements occur from time to time 

 in the unopened egg. They cannot however be of any 

 great extent until the fourteenth day, for up to this 

 time the embryo retains the position in which it was 

 first formed, viz. with its body at right angles to the 

 long axis of the egg. 



On the fourteenth day a definite change of position 

 takes place ; the chick moves so as to lie lengthways in 

 the egg, with its beak touching the chorion and shell 

 membrane where they form the inner wall of the 

 rapidly increasing air-chamber at the broad end (Chap. 

 I. p. 3). 



On the twentieth day or thereabouts the beak is 

 thrust through these membranes, and the bird begins 

 to breathe the air contained in the chamber. There- 

 upon the pulmonary circulation becomes functionally 

 active, and at the same time blood ceases to flow 

 through the umbilical arteries. The allantois shrivels 

 up, the umbilicus becomes completely closed, and the 

 chick piercing the shell at the broad end of the egg 

 with repeated blows of its beak, casts off the dried re- 

 mains of allantois, amnion and chorion, and steps out 

 into the world. 



