THE DEVELOPMENT OF THE VASCULAR SYSTEM 



241 



of the aorta passes ventrally through the mesentery as the vitelline artery 

 which enters the umbilical cord (Fig. 202). Still farther caudally the 

 paired umbilical (allantoic) arteries are given off from the aorta and pass 

 out into the umbilical cord (Fig. 202). 



The conditions which exist at this stage in the region of the aortic arches" 

 in mammalian embryos are indicative of the conditions which persist as a' 

 whole or in part throughout life in the lowest Vertebrates. The changes' 

 which occur in Mammals, however, are profound and the adult condition 

 bears no resemblance to the embryonic. Yet certain features in the adult 

 are intelligible only from a knowledge of their development. In the human ( 



Vent, aortic roots 



Ventral aortic trunk 



Gubclavian arteries 



Aorta 



FIG. 219. Diagram of the aortic arches of a Mammal. Modified from Ilochstettcr. 



embryo six aortic arches appear on each side. The first, second, third, and 

 fourth pass through the corresponding branchial arches. The fifth arch, 

 which is merely a loop from the fourth, seems to pass through the fourth 

 branchial arch. The sixth aortic arch passes through the region behind 

 the fourth branchial. All these arches are present in embryos of 5 mm. 

 (Fig. 218). In Fishes and larval Amphibians, where the branchial arches 

 develop into the gills, the aortic arches are broken up into capillary net- 

 works which ramify in the gills, and the ventral aortic root becomes the 

 afferent vessel, the dorsal aortic roots the efferent vessels. In the higher 

 Vertebrates and in man the aortic arches begin, at a very early period, to 



