The stage before the last stage (fifth) of the period 

 described (17 - 19th day) is characterized by the following 

 features (§ 13) . The yolk sac becomes empty and divided by 

 one or several deep folds. The quantity of the urinary 

 precipitates in the cavity of the chorion (allantoic cavity) 

 increases. The albumen is almost completely absent, while 

 the amniotic fluid also diminishes. The loops of the 

 intestine situated outside the abdominal cavity continue 

 retraction backwards, attracting the yolk surrounding the 

 mucous and vascular sacs. The feather rudiments are very 

 long, but the feathers do not yet appear. 



The last stage (twentieth and twenty-first day of 

 incubation; § 14) is considered the preparatory stage for 

 hatching. The fluid from the cavity of the amnion and 

 chorion has almost disappeared. The embryo occupies nearly 

 all of the egg cavity with the exception of the air sac. 

 The yolk sac is gradually pulled into the abdominal cavity. 

 It is possible to observe the stage when the yolk sac is 

 present half in the abdominal cavity and half outside it, and 

 they are connected by a narrow neck, situated in the umbilicus 

 At that time, the umbilical orifice becomes tightened and 

 begins healing. 



The arteries which connect the aorta with the pulmonary 

 arteries contract, forming a boat- like duct. 



The movement of the chick's head ruptures the chorion. 

 In this case, the end of the beak penetrates the air chamber, 

 which makes breathing possible. A first squeak accompanies 

 the breathing. Baer sometimes heard the cheep of the chick 

 still in the unbroken egg shell two days before hatching. 

 Sometimes the movement of the head breaks the shell, after 

 which breathing and the first squeak occur. After that, 

 breathing begins with atmospheric air; the vessels of the 



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