668 DEVELOPMENT OF THE CIRCULATORY APPARATUS. 



single vein, connecting the internal and external portions of the 

 vitelline circulation. 



The vessels belonging to this system are therefore called the 

 omphalo-mesenteric vessels, because a part of them (omphalic ves- 

 sels) pass outward, by the umbilicus, or " omphalos," to the umbili- 

 cal vesicle, while the remainder (mesenteric vessels) ramify upon 

 the mesentery and the intestine. 



At first, the circulation of the umbilical vesicle is more import- 

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

 appear accordingly as large trunks, of which the mesenteric ves- 

 sels are simply small branches. (Fig. 25-i.) Afterward, however, 

 the intestine rapidly enlarges, while the umbilical vesicle dimi- 

 nishes, and the proportions existing between the two sets of vessels 

 are therefore reversed. (Fig. 255.) The mesenteric vessels then 



Fig. 255. 



Diagram of EMBRYO AND ITS VESSELS; showing the second circulation. The pharynx, 

 oesophagus, and intestinal canal, have become further developed, and the mesenteric arteries have 

 enlarged, while the umbilical vesicle and its vascular branches are very much reduced in size. The 

 large umbilical arteries are seeu passing out to the placenta. 



come to be the principal trunks, while the omphalic vessels are 

 simply minute branches, running out along the slender cord of the 

 umbilical vesicle, and ramifying in a few scanty twigs upon its 

 surface. 



