382 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



for the embryo to sink into the yolk and that the amnion is to prevent its being 

 completely covered by this substance. It should be noted that the amnion 

 only occurs in the terrestrial forms, and that it results in the complete envelop- 

 ment of the embryo by a fluid medium which acts like a water cushion, tending 

 to prevent. injuries which might accompany the deposition of the egg on the 

 land. 



The homologue of the allantois is found in the urinary bladder of 

 the amphibia. It is an outgrowth from the ventral side of the hinder 

 end of the digestive tract and consists of a lining of entoderm, covered 

 externally with the splanchnic layer of the mesoderm — is developed 

 solely from the splanchnopleure — and projects into the coelom (fig. 

 404, lower). In its outgrowth it carries with it branches of the 

 hypogastric blood-vessels, the allantoic arteries and veins (iig. 

 329, usually there is but a single vein). As it develops, it extends 

 into the exocoelom, its distal end swelling into a large sac, connected 

 with the digestive tract by a slender stalk. Usually it extends to the 

 chorion and fuses with it, and the sac, flattening out, may gradually 

 extend until the embryo and the amnion are nearly enclosed by it. 

 In some cases the amnion expands and fuses with one side of the 

 allantois. 



In the sauropsida the allantois (and its covering chorion) comes 

 eventually to lie just beneath the shell, and, as the latter is very 

 porous and the allantois richly vascular, the latter is in position to 

 function in respiration. As the embryo increases in size, the side 

 walls of the body gradually close in, ventral to the embryo, until they 

 reach the stalks of the yolk sac and the allantois. In this way these 

 structures come to be connected with the body by a narrow band 

 (the navel or umbilical cord of mammals) in which the allantoic and 

 vitelline blood-vessels run. 



The mammals vary from this account of the allantois, but the differences 

 can be reconciled with the conditions in the sauropsida. In the majority of 

 the mammals there is the same union of allantois and chorion, and from the 

 union the peculiarly mammalian structure, the placenta, is formed. 



The placenta is a more or less intimate union of the allantoic 

 region of the embryo with the wall of the uterus of the mother. It 

 serves for the nutrition and respiration of the growing young. For 

 a comprehension of its structure a few details of the composition of 

 the uterus are necessary. The wall of the uterus has an outer coat, a 

 part of the peritoneum, inside of which is a thick muscular layer, this 

 being followed by a mucous layer (mucosa) which consists largely 



