THE ARTERIES AND VEINS. 183 



Tl ye cutaneous artery (Fig. 80, ad) is a branch of the fourth 

 efferent vessel, which arises very close to the pulmonary artery, 

 but independently of this, and at a rather later stage. 



5. Thejfein&. 



The veins, in the earlier stages of their development, are 

 chiefly characterised by their large size, and irregular lacunar 

 character. 



a. The vitelline veins are the first veins to be formed in the 

 body. They appear as irregular lacunee in the mesoblast of the 

 splanchnopleure, along the sides of the yolk-mUss and of the liver 

 diverticulum, and u nite in front to form the sinus venosus or 

 most posterior part of the heart ; th ey carry to the heart the 

 fo od matter absorbed from the yolk-mass. 



About, or shortly after, the time of hatching, the liver diver- 

 ticulum becomes more definitely bounded, and the vitelline and 

 he patic veins beco me distinct from one anothe r ; later still, by 

 folding of the wall of the hepatic diverticulum, the hepatic veins 

 are carried deeply into the substance of the liver (Pigs. 64 and 

 76), as already described in the section dealing with the deve- 

 lopment of the liver (p. 164). 



b. The sinus veno sus (Pig. 71, RS) is at first formed merely 

 by the union of the vitelline veins, but very early becomes a 

 definite transverse vessel, runnmg across tne Dody in close con- 

 tact with the anterior wall of the liver, and op ening into the 

 auric ular portion of the heart b y a large anterior apertu re. 



0. Th e, anterior cardinal veins are paired, and return blood 

 from all parts of the head, except the floor of the mouth. Each 

 is f ormed by the union, behind the ear, of two principal veins : — 

 the Jlugular vein, which returns blood from the brain and dorsal 

 part of tue head ; and the facial vei n, which runs more super- 

 ficially along the side of the head, ventral to the eye and ear. 



d. T he p osterior ca rdinal veins ( Pig. 84, vc) are also paired, 

 and a re m special relation with the head kidneys, which the y 

 c ompletely surround. ' They are of enormous size during the 

 early stages of tadpole life, when the head kidneys are function- 

 ally active, fo rming vascular networks which occupy the spaces 

 bet ween the tubules of the head kidneys. E ach posterior car - 

 d inal vein re ceives somatic veins from t he hinder part of the 

 body wall, and unites, in front, witti the anterior cardinal 



