CHAPTER XVII 



Reptilia 



Amnion and Allantois. — Comparison of Reptilia with Aves and 

 Mammalia shows that the first two Classes, sometimes grouped as 



Sauropsida^ are much more 

 closely related to each 

 other than either one is to 

 the Mammalia. The three 

 classes are alike in possess- 

 ing a structure called the 

 amnion. 



In the course of devel- 

 opment in the reptiles, birds 

 and mammals, called 

 Amniota, the embryo is 

 enclosed in a membranous 

 dome-like sac, the amnion, 

 which contains a fluid, the 

 amniotic liquor. A net- 

 work of blood vessels is 

 developed over the yolk-sac 

 which is an organ of res- 

 piration as well as of nutri- 

 tion. (Figure 162.) 



w\ '' f / ^^^ I ra^^Mi • vtM* -^" higher mammals, 



however, the allantois 

 effects respiration. The 

 allantois is a vascular sac- 

 like outgrowth from the 

 Fig. 162. Vertebrate embryos with their hinder part of the embry- 

 membranes. A, reptile or bird; 5, placental onic intestine. It is pres- 

 mammal. In A the yolk sac is functional and ent in Amphibia but is very 

 the allantois respiratory; in B the yolk sac is 5^1^11. 'XV^ fishes and am- 



functionless and the allantois becomes the , ., . , , • u ^u „ii„„ 

 , . , .,■ I J /Ar phibia, lackmg both allan- 



nutntive placenta and umbihcal cord. (Arter ^ , .° 



Wilder, History of the Human Body. Courtesy tois and amnion, are some- 

 of Henry Holt & Co.) times called Anamniota. 



302 



