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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 

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

 terior cardinal veins. The renal portal veins consist of 

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

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

 edian cardinal vein, which represents the hinder part of the 

 riginal posterior cardinal veins. 



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

 all, 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 their 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 

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

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

 ,ter form a definite tube opening proximally into the left side 

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

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

 ase 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 



to 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 



