224 THE SIXTH DAY. [CHAP. VIII. 



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. § 2). 



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. Thereupon 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 

 remains of allantois, amnion and chorion, and steps out into 

 the world. 



