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TEXT-BOOK OF EMBRYOLOGY. 



embraces the entire yolk sac, and this fact indicates a precocious development of 

 the vascular anlagen. In an embryo of 3.2 mm. the vitelline circulation is 

 complete. A study of Fig. 215 will assist in understanding the similarities 

 between the early form of circulation in the human embryo and the other forms 

 which have been considered. It is thus seen that the earliest form of circulation 

 in man, as well as in lower forms, is associated with the yolk sac. 



In animals below the Mammals, where a large amount of yolk is present, 

 the vitelline circulation is of great importance in supplying the growing embryo 

 with nutritive materials. In Mammals, where there is little yolk present, the 



Gut 



Umbilical vein 



Amnion 



Allantois 



Yolk stalk 



Umbilical artery 

 Umbilical vein 



Amnion 



Chorionic villi 



FIG. 217. Diagram of the umbilical vessels in the belly stalk and chorion. Kollmann's Atlas. 



vitelline circulation is of short duration and minor importance, yet those por- 

 tions of the vessels inside the embryo play a part in the further development of 

 the vascular system. Again, in Reptiles and Birds, a second circulation, as it 

 were', develops in connection with the allantois, and persists during incubation, 

 since the allantois is a reservoir for the waste products of the body. In Mam- 

 mals, however, the allantois is rudimentary, its place being taken by the pla- 

 centa which establishes the communication between the embryo and the mother; 

 and the vessels which correspond to the allantoic vessels in Reptiles and Birds 

 become associated with the placental circulation. Two arteries, one on each 



