EMBRYOLOGY. 275 



The ureters are developed from the kidneys and pass downward 

 to be connected with the bladder. 



The circulatory apparatus assumes three forms at different periods 

 of life, all having reference to the manner in which the embryo 

 receives nutritive matter and is freed of waste products. 



The vitelline circulation appears first and absorbs nutritive ma- 

 terial from the vitellus. It is formed by blood-vessels which emerge 

 from the body and ramify over a portion of the vitelline membrane, 

 constituting the area vasculosa. The heart, lying in the median 

 line, gives off two arches, which unite to form the abdominal aorta, 

 from which two large arteries are given off, passing into the vas- 

 cular area ; the venous blood is returned by veins which enter the 

 heart. These vessels are known as the omphalo-mes enteric arteries 

 and veins. The vitelline circulation is of short duration in mam- 

 mals, as the supply of nutritive matter in the vitellus soon becomes 

 exhausted. 



The placental circulation becomes established when the blood- 

 vessels in the allantois enter the villous processes of the chorion and 

 come into close relationship with the maternal blood-vessels. The 

 circulation lasts during the whole of intra-uterine life, but gives 

 way at birth to the adult circulation, the change being made possible 

 by the development of the circulatory apparatus. 



The heart appears as a mass of cells coming off from the anterior 

 portion of the intestine ; its central part liquefies, and pulsations 

 soon begin. The heart is at first tubular, receiving posteriorly the 

 venous trunks and giving off anteriorly the arterial trunks. It soon 

 becomes twisted upon itself, so that the two extremities lie upon 

 the same plane. 



The heart now consists of a single auricle and a single ventricle. 

 A septum, growing from the apex of the ventricle, divides into 

 two cavities, a right and a left. The auricles also become partly 

 separated by a septum, which is perforated by the foramen ovale. 

 The arterial trunk becomes separated, by a partition, into two 

 canals, which become, ultimately, the aorta and the pulmonary 

 artery. The auricles are separated from the ventricles by incom- 

 plete septa, through which the blood passes into the ventricles. 



Arteries. The aorta arises from the cephalic extremity of the 

 heart and divides into two branches, which ascend, one on each side 

 of the intestine, and unite posteriorly to form the main aorta ; pos- 



