DEVELOPMENT 



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over the back of the embryo till they form a closed sac, 

 called the amnion. It is filled with a thin liquid, which 

 serves to protect the embryo. Meanwhile, another im- 

 portant organ is forming on the other side. From the 



FlG. 365. Embryo in a Hen's Egg during the first five days, longitudinal view : 

 A, hypoblast ; B, lower layer of mesoblast; C, upper layer of mesoblast and epiblast 

 united, in the last figures forming the amniotic sac ; 'D, vitelline membrane ; 

 e, thickened blastoderm, the first rudiment of the dorsal part (in the last figure it 

 marks the place of the lungs); h, heart; a, b, its two chambers ; c, aortic arches ; 

 m, aorta; /, liver; /, allantois. 



