THE VEINS. 309 



the body of the embryo, excepting the alimentary canal and the 

 liver. The anterior cardinal veins persist throughout life as 

 the jugular veins, and are joined at an early stage by pectoral 

 veins from the wing, and vertebral veins from the head and neck. 

 The posterior cardinal veins remain of large size so long as the 

 Wolffian bodies are in functional activity ; but when these 

 begin to diminish in size, on the appearance of the permanent 

 kidneys, the posterior cardinal veins also shrink up, and ulti- 

 mately disappear. 



The Cuvierian veins, as already noticed, persist and become 

 the anterior venas cavas of the adult bird. 



b. The System of the Posterior Vena Cava. 



The meatus venosus. The heart, as already described, is 

 formed by the union of the right and left vitelline veins (Fig. 

 127, vv), which lie along the edge of the fold of the splanchno- 

 pleure, marking the posterior limit of the fore-gut (cf. Fig. 112). 



The vitelline veins remain at the edge of the splanchno- 

 pleuric fold, and consequently travel backwards with this fold as 

 the embryo becomes more definitely constricted from the yolk- 

 sac. This shifting backwards of the point of meeting of the 

 right and left vitelline veins causes, first, a lengthening of the 

 heart ; and then, after the heart is definitely established, the 

 formation of a median vitelline vein lying posteriorly to the 

 heart, between the hinder end of the heart and the point of 

 meeting of the two vitelline veins. 



This median vitelline vein may be divided into an anterior 

 part or sinus venosus (Fig. 128, RS), which really forms the 

 posterior chamber of the heart and receives the Cuvierian veins ; 

 and a posterior part or meatus venosus, VE. 



The meatus venosus has, from the first, very close relations 

 with the liver. The two primary liver diverticula lie one on 

 each side of it, and as the liver increases in size it completely 

 surrounds the vein. Blood-vessels appear in the substance of 

 the liver mass, and soon acquire openings into the meatus 

 venosus. At first these vessels are very irregularly disposed, 

 but by about the fifth day a definite arrangement can be made 

 out. The meatus venosus on entering the liver gives off afferent 

 hepatic vessels (Fig. 128, vo), which open into the capillary 



