DEVELOPMENT OF THE VASCULAR SYSTEM. 689 



vessels. Those within the abdomen supply the mesentery and intestine ; 

 while the remainder pass outward and ramify on the walls of the um- 

 bilical vesicle (Fig. 236). At first there are, as above mentioned, two 

 omphalo-mesenteric arteries emerging from the body, and two omphalo- 

 mesenteric veins returning to it ; but afterward the two arteries are 

 replaced by a common trunk, while a similar change takes place in 

 the veins. Subsequently, therefore, there remain but one artery and 

 one 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 



Diagram of the EMBRYO AND ITS VESSELS, showing the second or placental circulation. The intes- 

 tine has become further developed, aud the mesenteric arteries have enlarged, while the umbili- 

 cal vesicle and its vascular branches are reduced in size. The large umbilical arteries are seen 

 passing out to the placenta. 



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. 236). Afterward the intestine enlarges, while the um- 

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

 vessels is reversed. The mesenteric vessels then come to be the prin- 

 cipal trunks, while the omphalic vessels are minute branches, running 



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