THE VASCULAR SYSTEM 347 



the allantois. This membrane is itself the amphibian urinary 

 bladder extended beyond the limits of the embryo, and there 

 is little doubt that the two veins which lie along its sides and 

 enter the liver, are the primary lateral elements which in adult 

 amphibians fuse to form the median abdominal vein. In 

 the later history of the umbilical veins, the right one becomes 

 early reduced, and in advanced embryos the left one alone re- 

 mains; this collects all the blood from the entire allantois, 

 enters the body at the umbilicus, and conveys the blood from 

 that point to the liver. 



At birth, in the case of the mammal, and upon hatching, 

 in reptiles and birds, the extra-embryonal portion of the al- 

 lantois, together with its blood-vessels, becomes pinched off 

 at the umbilicus, but the umbilical vein, extending from the. 

 anterior body wall to liver, is retained as a ligament (lig. teres 

 s. hepato-umbilicale). 



The portal vein, previously described, which conveys the 

 blood from the intestine to the liver, is a constant factor 

 in vertebrate circulation from cyclostomes to mammals, and 

 as it is essentially similar in all cases there is but little his- 

 tory shown by the comparison of adult forms. The early 

 embryonic development shows, however, the method by which 

 this portal system becomes established, and is thus valuable 

 in explaining the relation between the original morphological 

 elements and the adult structures. The formation of the 

 hepatic-portal system occurs always in connection with the 

 two first veins that appear, the vitelline or omphalo-mesenteric, 

 that lead in from the yolk and unite just posterior to they 

 heart. The intestinal canal runs between them, and from its 

 ventral aspect the liver buds out in the form of a connected 

 group of diverticula, which surround the veins in question and 

 cause them to develop a capillary net-work. In fishes and 

 amphibians the process is a fairly simple one, but in Saurop- 

 sida and Mammalia the matter becomes somewhat more com- 

 plicated by the addition to tjiis very region of the two um- 

 bilical veins, which come in from the allantois. This develop- 

 ment, in its more complex form, is shown in Figs. 97 and 99, 



