AVES. 199 



allantois and the bloodless serous membrane and amnion. The 

 amnion, whose fluid during the later days of incubation rapidly 

 diminishes, is continuous at the umbilicus with the body-walls 

 of the embryo. The serous membrane (or outer primitive 

 amniotic fold) is, by the completion of the cleavage of the meso- 

 blast and the withdrawal of the yolk-sack, entirely separated 

 from the embryo. The cavity of the allantois, by means of its 

 stalk passing through the umbilicus, is of course continuous with 

 the cloaca. 



When the chick is about to be hatched it thrusts its beak 

 through the egg-membranes and begins to breathe the air con- 

 tained in the air chamber. Thereupon the pulmonary circulation 

 becomes functionally active, and at the same time blood ceases 

 to flow through the allantoic 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 serous 

 membrane, and steps out into the world. 



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