DEVELOPMENT OF THE ARTERIAL SYSTEM. 669 



In the mean time, the allantois is formed by a protrusion from 

 the lower extremity of the intestine, which, carrying with it two 

 arteries and two veins, passes out by the anterior opening of the 

 body, and comes in contact with the external membrane of the 

 egg. The arteries of the allantois, which are termed the umbilical 

 arteries, are supplied by branches of the abdominal aorta ; the um- 

 bilical veins, on the other hand, join the mesenteric veins, and 

 empty with them into the venous extremity of the heart. As the 

 umbilical vesicle diminishes, the allantois enlarges ; and the latter 

 soon becomes converted, in the human subject, into a vascular 

 chorion, a part of which is devoted to the formation of the placenta. 

 (Fig. 255.) As the placenta soon becomes the only source of nutri- 

 tion for the foetus, its vessels are at the same time very much 

 increased in size, and preponderate over all the other parts of the 

 circulatory system. During the early periods of the formation of 

 the placenta, there are, as we have stated above, two umbilical 

 arteries and two umbilical veins. But subsequently one of the 

 veins disappears, and the whole of the blood is returned to the body 

 of the fcetus by the other, which becomes enlarged in proportion. 

 For a long time previous to birth, therefore, there are in the umbili- 

 cal cord two umbilical arteries, and but a single umbilical vein. 



Such is the second, or placental circulation. It is exchanged, at 

 the period of birth, for the third or adult circulation, in which the 

 blood which had previously circulated through the placenta, is 

 diverted to the lungs and the intestine. These are the organs 

 upon which the whole system afterward depends for the nourish- 

 ment and renovation of the blood. 



During the occurrence of the above changes, certain other altera- 

 tions take place in the arterial and venous systems, which will now 

 require to be described by themselves. 



Development of the Arterial System. At an early period of deve- 

 lopment, as we have shown above, the principal arteries pass off 

 from the anterior extremity of the heart in two arches, which curve 

 backward on each side, from the front of the body toward the 

 vertebral column, after which they again become longitudinal in 

 direction, and receive the name of "vertebral arteries." Very soon 

 these arches divide successively into two, three, four, and five 

 secondary arches, placed one above the other, along the sides of 

 the neck. (Fig. 256.) These are termed the cervical arches. In the 

 fish, these cervical arches remain permanent, and give off from their 

 convex borders the branchial arteries, in the form of vascular tufts, 



