752 TEXT-BOOK OF PHYSIOLOGY 



additional material in the same way. The period during which it does so 

 is, however, very limited. 



The Vitelline Circulation. -The vitellme circulation, which m oviparous 

 animals, e.g., the chick, is of primary importance because of the large 

 amount of food stored in the vitellus or yolk, is in mammals of relatively 

 slight importance because of the limited supply of ;food in the vitellus. It 

 is nevertheless present in early stages. 



The Allantoic Circulation which in oviparous animals is also of primary 

 importance in the latter half of the incubation period both as an absorption 



da. 



FIG. 331. THE FETAL CIRCULATION, ao. Aorta, a.pu. Pulmonic artery, aw. Umbilical 

 artery, da. Ductus arteriosus. dv. Ductus venosus. int. Intestine, vci and vcs. Inferior and 

 superior venae cavae. vh. Hepatic vein. vp. Venaportae. v. pu. Pulmonic vein. vu. Umbilical 

 vein. (From Kollmann.) 



and respiratory apparatus is also present in mammals to a slight extent 

 but it is merely a transition stage in the development of placental circulation. 

 The Placental Circulation. The development of the fetal or placental 

 circulatory apparatus by which the fetus obtains its food supply and neces- 

 sary oxygen and frees itself from carbon dioxid has been alluded to in a 

 foregoing paragraph relating to the formation of the placenta. After the 

 blood-vessels of the embryo, the umbilical arteries and vein have come into 

 histologic and physiologic relations with the uterine blood-vessels, the 

 nutritive materials and the oxygen are derived entirely from the maternal 



