vi VEINS OF LACEETA 423 



network (Fig. 195, D) and then in the region in front of the network 

 (Fig. 195, E). Kesults of these changes are (1) that the hepatic 

 network receives its blood supply by a single afferent vessel — the 

 hepatic portal vein — which curves round the gut and is derived in 

 great part from the left vitelline vein — and (2) that its blood drains 

 away to the heart by a single efferent vessel — the hepatic vein — 

 derived from the front end of the original right vitelline vein. 



At a comparatively early stage in development a direct channel 

 becomes established, by the widening out of the venous spaces along 

 the middle of the hepatic network, so that a considerable proportion 

 of the blood is able to pass forwards from the portal vein through 

 the liver without actually traversing the network itself. This 

 channel — the ductus venosus (Fig. 195, G, d.v) persists till nearly 

 the period of hatching but then becomes obliterated so that all the 

 portal blood has to traverse the hepatic network. 



The posterior vena cava makes its appearance as a gradually 

 widening channel through the hepatic network towards its right side 

 (Fig. 195, D, E, p.v.c). This portion of the liver becomes prolonged 

 backwards as a slender lobe ensheathing a prolongation of the blood 

 channel- mentioned. This prolongation fuses at its tip with the tip 

 of the right opisthonephros, continuity becomes established between 

 the venous spaces of the two organs and finally, as in the Amphibian, 

 the liver tissue disappears over a large stretch of the slender lobe 

 already mentioned so that the vena cava is now for a considerable 

 length free from either liver or kidney. 



At an early stage a branch of the vitelline vein develops close to 

 its front end. This is the allantoic or umbilical vein (Fig. 195, 

 r.all and l.all). These -veins soon become asymmetrical, the left 

 for a time being smaller than the right (Fig. 195, D, E). A little 

 later however the left vein establishes a connexion with the hepatic 

 network (Fig. 195, F); the- portion of the vein posterior to this 

 connexion becomes much widened, and the blood-stream from it 

 courses by an enlarged direct channel of the network into the 

 posterior vena cava (Fig. 195, F, G). The blood-stream being diverted 

 through this channel, the portion of the left allantoic vein in front 

 of it shrinks in size and disappears, as does the whole of the right 

 allantoic vein (Fig. 195, F, G). The result is that there persists a 

 single (left) allantoic vein which drains the blood from the allantois 

 into the posterior vena cava near its front end. The allantoic vein 

 increases in size with the allantois but becomes obliterated at the 

 time of hatching when the allantois is cast off. The mesenteric vein 

 develops as a branch of the portal vein (left vitelline vein) a short 

 distance behind its entry into the liver : it increases in size as the 

 vitelline diminishes with the consumption of the yolk and eventu- 

 ally it alone persists as the peripheral portion of the definitive portal 

 vein of the adult. 



The subintestinal vein is apparently present only in its post-anal 

 portion which persists as the caudal vein of the adult. In front 



