REPTILES AND BIRDS 



385 



out over the inner surface of the egg membrane. The egg membrane 

 is in turn in close contact with the shell. Into this chorion extend the 

 allantoic blood vessels, which form a rich capillary network (Fig. 271). 

 Since the shell is in all cases sufficiently porous to permit of the passage 

 of air, respiration is carried on through it. This explains why it is 

 fatal to the developing embryo if the shell is covered by any material 

 which closes the pores and makes it impervious to the passage of gases. 



,Aflani-oi5 

 / anc^ blood 

 vessels 



Yolk circulcff-ion 



Fig. 271. — Diagram of a stage in the development of a bird's egg, later than showTi in 

 Fig. 256, and indicating the circulation; arteries unshaded, veins black. Arrows show 

 direction of blood flow. {From Wilder, "History of the Human Body," by the courtesy of 

 Henry Holt & Company.) 



When the young animal hatches from the egg, the connections of 

 the amnion and allantois with the body are broken, and these are left 

 behind in the empty shell. Though not much tissue is sacrificed, this 

 presents a contrast to the condition seen in previous types, in which 

 all of the egg cell became part of the body of the animal to which it 

 gave rise. 



412. Body Coverings. — The birds and reptiles are distinguished 

 particularly by the body coverings. Reptiles possess horny epidermal 

 scales which usually overlap, and birds are covered by feathers which are 

 similar in their origin and in their general mode of development to scales 

 and which are, therefore, looked upon as modifications of them. 



