DEVELOPMENT OF THE VASCULAR SYSTEM. 



793 



Fig. 299. 



Diagram of the YOUNG EMBRYO AND 

 ITS VESSELS, showing the circulation 

 of the umbilical vesicle, and also that of 

 the allantois, beginning to be formed. 



Placental Circulation. After the umbilical vesicle has been formed 

 by the process already described (page 738), a part of the vitellus re- 

 mains included in it, while the rest is retained in the abdomen and 

 inclosed in the intestinal canal. As 

 these two organs (umbilical vesicle 

 and intestine) are originally parts of 

 the same vitelline sac, they remain 

 supplied by the same vascular sys- 

 tem, namely, the omphalo-mesenteric 

 vessels. Those which remain within 

 the abdomen of the foetus supply the 

 mesentery and intestine; but the 

 larger trunks pass outward, and 

 ramify upon the walls of the um- 

 bilical vesicle. (Fig. 299.) At first 

 there are, as above mentioned, two 

 omphalo-mesenteric arteries emerg- 

 ing from the body, and two omphalo- 

 mesenteric veins returning to it ; but 

 afterward the two arteries are re- 

 placed by a common trunk, while a 

 similar change takes place in the two veins. Subsequently, therefore, 

 there remains but a single artery and a single vein, connecting the 

 internal and external portions of the vitelline circulation. 



The vessels belonging to this system are called the omphalo-mesen- 

 teric vessels, because a part of them (omphalic vessels) pass outward, 

 by the umbilicus, or " omphalos," to the umbilical vesicle, while the 

 remainder (mesenteric vessels) ramify upon the mesentery and the 

 intestine. 



At first, the circulation of the umbilical vesicle is more important 

 than that of the intestine ; and the omphalic artery and vein appear 

 accordingly as large trunks, of which the mesenteric vessels are small 

 branches. (Fig. 299.) Afterward the intestine enlarges, while the um- 

 bilical vesicle diminishes ; and the proportion between the two sets of 

 vessels is therefore reversed. The mesenteric vessels then come to be 

 the principal trunks, while the omphalic vessels are minute branches, 

 running out along the stem of the umbilical vesicle, and ramifying in a 

 few scanty twigs upon its surface. (Fig. 300). 



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

 abdominal aorta ; the umbilical veins, on the other hand, join the mesen- 

 teric veins, and empty with them into the venous extremity of the heart. 

 As the umbilical vesicle diminishes, the allantois enlarges ; and the lat- 

 ter is converted, in the human subject, into a vascular chorion, part of 

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