THE LATER DEVELOPMENT OF THE FROG 189 



very short region derived from the original right posterior 

 cardinal vein, and finally, the entire posterior section is formed 

 from the median channel, derived from the fused right and left 

 posterior cardinal veins. The renal portal veins consist of 

 two sections: a posterior part is derived from the iliac vein, 

 and an anterior part is formed from the lateral channel of the 

 median cardinal vein, which represents the hinder part of the 

 original posterior cardinal veins. 



A pair of lateral veins develops late, in the ventral abdominal 

 wall, for a time opening directly into the sinus venosus. Pos- 

 teriorly these connect with the iliac veins, and then continue, 

 fusing together medially. The anterior portions of these ves- 

 sels then lose thair connection with the sinus venosus and the 

 anterior part of the right vessel disappears entirely, the left 

 vein forming a new connection with the hepatic portal vein, 

 when it is known as the anterior abdominal vein. 



The rudiments of the pulmonary veins are indicated very 

 early (about 6 mm.) as proliferations of the endot helium on 

 the dorsal side of the sinus venosus (Federow). These cells 

 later form a definite tube opening proximally into the left side 

 of the auricle, and distally leaving the wall of the sinus venosus 

 and passing dorsally to the rudiments of the lungs. At the 

 base of the lung it bifurcates, each branch passing along the 

 medio-ventral side of each lung rudiment. Later, when the 

 lungs become functional the pulmonary veins discharge into 

 the left auricle. 



5. The Lymphatic System and Spleen 



The first indications of this system appear shortly before 

 hatching. In the larva of 6.5 mm. (Knower) a single pair of 

 anterior lymph hearts is present, as small sac-like outgrowths 

 of a pair (usually the fourth) of intersegmental veins (i.e., veins 

 running between the fourth and fifth myotomes, and opening 

 into the posterior cardinal veins at the posterior limit of the 

 pronephros). These hearts lie between the peritoneum and 

 the integument, below the level of the myotomes. The 

 endothelial wall of the lymph hearts is continuous with that 



