796 



COMPARATIVE ANATOMY 



- card.p. 



omph m.d, - 



Fig. 451. 



ymph.m.s. 



omph.- 



yolk, the presence of a large food supply exercises a modifying influence 

 on these ventral veins. A pair of large vitelline veins runs out into the 

 yolk sac, over the yolk, from the junction of the omphalomesenteric and 

 the subintestinal veins to play a large part in the transfer of material to 



the growing embryo (Fig. 284). The distal 

 parts of these veins follow the margin of the 

 yolk sac, forming a tube (sinus terminalis), 

 into which smaller veins empty. Blood is 

 brought to the yolk by the omphalo-mesen- 

 teric arteries. These arteries are also dis- 

 tributed to the yolk sac and divide up dis- 

 tally into a network of capillaries which 

 connect distally with the vitelline veins. The 

 blood is carried by these vitelline veins to 

 the liver and through the portal circulation 

 to the lieart. In the mammals a similar 

 vitelline circulation is developed, but here 

 the yolk sac contains no yolk, and so is of 

 minor importance and soon lost. 



In amniotes there is an outgrowth, the 

 allantois, which arises as a diverticulum 

 from the hinder end of the alimentary canal. 

 This increases in extent by growing down- 

 ward and carrying the ventral body wall be- 

 fore it. Allantoic arteries (branches of the hypogastric arteries) extend 

 into it and are connected by capillaries with umbilical veins which arise 

 from the subintestinal vein behind the vitelline veins. This forms an 

 allantoic circulation which is both respiratory and nutritive in character. 

 In the reptiles, both of the umbilical veins persist through foetal life, 

 while in birds and mammals, one aborts, leaving the other as the efferent 

 vessel of the allantois. With the end of foetal life (at hatching or at 

 birth) both the vitelline and the allantoic circulations disappear, leaving 

 only inconspicuous rudiments. 



Anterior Cardinal Veins (Superior Jugular or Jugular) : 



(The inferior jugulars are found only in fishes and salamanders, 

 where they drain lateral and ventral branchial regions.) The superior 

 jugular vein lies dorsal to the gill-clefts and returns blood from the dor- 

 sal regions of the head. 



Post Cardinal Veins (Figs. 450, 456). 



These are very clearly related in development with the excretory 

 system and lie dorsal to the coelom and dorsal to the nephridial arteries. 

 Nearly all of the thoracic portion of the post cardinal veins soon disap- 

 pears in the higher forms, while a supra cardinal system develops, as 

 shown in the figures. This supra cardinal system in turn disappears with 



Diagram showing development of 

 the mammalian hepatic portal 

 system. The omphalo-mesenteric 

 and the umbilical veins are reduced. 

 card.a. and card.p., anterior and 

 posterior cardinals ; d, intestine ; 

 d.ar., ductus venosus (Arantii) : I, 

 liver ; omph.m.d. and omph.m.s., 

 right and left omphalo-mesenteric 

 veins ; umb.d. and umb.8., right and 

 left umbilical veins. (After Hoch- 

 stetter. ) 



