690 REPRODUCTION. 



out to the umbilical vesicle, and ramifying- in a few scanty twigs upon 

 its surface (Fig. 237). 



In the meantime, 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 from the abdomen, and comes in contact 

 with the external membrane of the egg. The arteries of the allantois, 

 termed the umbilical arteries, are supplied by branches of the abdom- 

 inal aorta ; while the venous trunks returning from it, or the umbilical 

 veins, join the mesenteric veins, and empty with them into the venous 

 extremity of the heart. As the umbilical vesicle diminishes, the allan- 

 tois enlarges ; and the latter is converted, in the human subject, into 

 a vascular chorion, which serves for the formation of the placenta 

 (Fig. 237). As the placenta soon becomes the only source of nutrition 

 for the foetus, its vessels increase in size, and preponderate over all 

 other parts of the circulatory system. During the early periods of its 

 formation there are, as above mentioned, two umbilical arteries and 

 two umbilical veins. Subsequently one of the veins disappears, while 

 the other becomes enlarged in proportion, and returns the whole of the 

 blood from the placenta to the foetus. For a long time previous to 

 birth the umbilical cord contains therefore two umbilical arteries, and 

 but one umbilical vein. 



Adult Circulation. The placental circulation is exchanged at birth 

 for the third or adult circulation. This is distinguished by the disap- 

 pearance of the placenta and the vessels connected with it, and by the 

 entrance into activity of the lungs and the alimentary canal, as the 

 organs of nutrition and aeration. A large proportion of the blood is 

 accordingly turned away from its former channels, and distributed to 

 new organs. This change is comparatively rapid. The previous tran- 

 sition, from the vitelline to the placental circulation, was gradual ; the 

 umbilical vesicle diminishing simultaneously with the enlargement of 

 the placenta, and the two organs, with their blood-vessels, coexisting 

 for a certain period. But at birth the placenta is suddenly withdrawn 

 from the circulatory system and replaced in functional activity by the 

 lungs and the alimentary canal. 



This change, however, has been already provided for by the gradual 

 development of the necessary organs, and by corresponding alterations 

 in both the arterial and venous systems. 



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

 opment, the arterial trunks, after passing off from the anterior extremity 

 of the heart, curve backward, as already described (page 688), in two 

 sets of lateral branches, toward the vertebral column, after which they 

 reunite, to form the " vertebral arteries." The curved branches, em- 

 bracing the sides of the neck, are called the cervical arches. They 

 pass through the substance of the " visceral folds," already described 

 (page 677), and are separated from each other by the intervening cer- 

 vical fissures. In the embryo chick, according to Foster and Balfour, 

 three cervical arches, in the three upper visceral folds, have been formed 



