322 HISTORY OF THE HUMAN BODY 



the figure), which collect the blood sent to the growing- tis- 

 sues of the embryo by the arteries and return it into the main 

 channel. The anterior and posterior cardinals of each side 

 unite opposite the heart and form a lateral vessel, the duct of 

 Cuvier (ductus Cuvieri), which enters the heart from the 

 side immediately after its formation through the union of the 

 two vitelline veins. In this system of vessels is seen the first 

 systemic venous system, the function of which is to collect 

 from the body the blood supplied it by the arteries and return 

 it to the heart. 



A considerable advance is seen in Fig. 89, B, which repre- 

 sents a somewhat older embryo. The heart has increased both 

 in caliber and in length, which has caused it to assume a some- 

 what contorted attitude, the preluda to those later changes 

 which will result in the formation b^^compact organ with 

 definite compartments. To the single^arterial loop which 

 forms the first arterial arch in the gill region others have been 

 added in a posterior direction, the general method of forma- 

 tion being shown by the last one, in this figure the 5th. The 

 appearance of limbs has caused the development of arteries to 

 supply them, subclavians for the anterior, and iliacs for the 

 posterior; these are duplicated by veins associated with the 

 cardinal system. 



At about this stage a striking change, but one the signifi- 

 cance of which is mainly embryonic, consists in the develop- 

 ment of the bag-like allantois with its accompanying blood- 

 vessels, the allantoic (or umbilical) veins and arteries (see 

 Fig. 17). This appears indeed in amphibians as an evagina- 

 tion from the ventral wall of the cloaca, where it functions 

 as a urinary bladder, but here it never surpasses the limits of 

 the body ; in Sauropsida and Mammalia, however, it develops 

 into an enormous extra-embryonal organ of great functional 

 importance. It begins in the form of a small sac that pushes 

 its way out from the embryo, and is supplied with two arteries 

 from the posterior aorta, and two veins which enter the heart 

 in association with the vitelline vein, but it soon increases 

 greatly in size, and its blood-vessels increase proportionately. 



