842 EMBRYOLOGY 



tion and growth. In mammals, however, nutritive matter is absorbed 

 almost exclusively from the mother, by simple imbibition, before the pla- 

 cental circulation is established, and by the placental vessels at a later 

 period. The vitelline circulation is therefore not important, and the 

 vessels disappear with the atrophy of the umbilical vesicle. 



The area vasculosa in mammals consists of vessels coming from the 

 body of the embryo, forming a nearly circular plexus in the substance 

 of the vitellus. The vessels of this plexus open into a sinus at the 

 border of the area, called the sinus terminalis (see Figs. 243 and 244). 



In examining the ovum when the area vasculosa is first formed, the 

 embryo is seen lying in the direction of the diameter of the nearly cir- 

 cular plexus of bloodvessels. The plexus surrounds the embryo, except 

 at the cephalic extremity, where the terminal sinuses of the two sides 

 curve downward toward the head to empty into the omphalo-mesenteric 

 veins. As the umbilical vesicle is separated from the body of the em- 

 bryo, it carries the plexus of vessels of the area vasculosa with it, the 

 vessels of communication with the embryo being the omphalo-mesen- 

 teric arteries and veins. As these processes are going on, the great 

 central vessel of the embryo becomes enlarged and twisted on itself 

 at a point just below the cephalic enlargement of the embryo, between 

 the inferior extremity of the pharynx and the superior cul-de-sac of the 

 intestinal canal. The excavation which receives this vessel is called the 

 fovea cardiaca. Simple undulatory movements take place in the heart 

 of the chick at about the middle of the second day ; but there is not at 

 that time a regular circulation. At the end of the second day or the 

 beginning of the third, the currents of the circulation are established. 

 The time of the first appearance of the circulation in the human embryo 

 has not been accurately determined. 



In the arrangement of the vessels for the first circulation in the 

 embryo, the heart is in the median line and gives off two arches which 

 curve to either side and unite into a single central trunk at the spinal 

 column below. These are the two aortae, and the single trunk formed 

 by their union becomes the abdominal aorta. The two aortic arches 

 only one of which is permanent are sometimes called the inferior 

 vertebral arteries. These vessels give off a number of branches which 

 pass into the area vasculosa. Two of these branches, however, are 

 larger than the others, pass to the umbilical vesicle and are called the 

 omphalo-mesenteric arteries. In the embryo of mammals, there are at 

 first four omphalo-mesenteric vems, two superior, which are the larger, 

 and two inferior ; but as development advances, the two inferior veins 

 are closed, and there are then two omphalo-mesenteric arteries and two 

 omphalo-mesenteric veins. At about the fortieth day, one artery and 



