the embryo. These vessels had been named "umbilical 

 arteries" because they go through the umbilicus, but in 

 Baer's opinion they would better be called the vessels of 

 the urinary sac. The external half of the urinary sac 

 represents the embryonic organ of respiration. It firmly 

 adjoins the shell emembrane, then gradually separates from 

 the shell and forms the chorion. The internal half of the 

 urinary sac becomes thin and fits close to the amnion and 

 yolk sac. 



Baer summarized changes in the incubated egg as follows. 

 In the process of incubation the quantity of albumen decreases 

 partly from evaporation and partly as a result of its use 

 by the embryo; thus the volume of the air chamber enlarges 

 at the blunt end of the egg. The yolk mass at first increases, 

 then decreases as the embryo uses it; the membranes of the 

 yolk and chalazae disappear. The middle part of the rudiment 

 is transformed into the embryo, and the periphery into the 

 blastoderm covering the yolk. Upon separation of the embryo 

 from the blastoderm, the umbilicus forms, and the extra- 

 embryonic blastoderm forms the yolk sac hanging from the 

 embryo. In the latter, the blood-carrying vessels of the 

 yolk sac branch and become the vessels of the mesentery along 

 the yolk duct. The animal layer of the rudiment membrane 

 forms two vesicles (or membranes) , the amnion and the serous 

 vesicle, of which only the amnion remains to the end of 

 development. From the vegetative region of the embryo, the 

 urinary sac, rich in vessels, protrudes and is gradually 

 overgrown by the embryo with its appendages, the yolk sac and 

 the amnion. The external part of the urinary sac forms the 

 chorion adjoining the shell membrane. The parts of the 

 extra- embryonic formations (albumen, yolk membranes, chalazae, 

 the yolk itself, the peripheral zone of the blastoderm, the 

 internal half of the urinary sac) are reduced at different 

 times. Only the embryo develops and grows continuously. 



At the end of incubation, the yolk sac enters through the 

 umbilicus to the embryonic abdominal cavity, where the remains 

 of the yolk are used for some weeks after hatching. During 

 the nineteenth and twentieth days of incubation, the umbilicus 

 narrows, which leads to blockage of blood circulation in the 

 umbilical vessels. The chicken tries to breathe with its 

 lungs by penetrating its beak into the air chamber or by 

 breaking the shell. Then the movement of the blood in the 



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