THE CIRCULATORY SYSTEM 



599 



ORIGIN OF THE CIRCULATORY SYSTEM AND THE BLOOD 



About the time of hatching, outgrowths of the dorsal aorta, just 

 back of the pharyngeal region, extend laterally into the region of the 

 pronephros or head kidney. These later become very large and form 

 the vascular glomi of the kidney, traces of which remain long after the 

 pronephros itself has disappeared. 



The origin of the blood itself is by no means well established in the 

 frog. The student should review the work on the chick in this respect. 

 All we can say in regard to the frog is that when the embryo is almost 

 ready for hatching, that is, when it is four to 4.5 millimeters in length, 

 irregular spaces appear in the mesenchyme and splanchnic mesoderm, 

 which later form continuous vessels. The corpuscles may arise as out- 

 growths from the endothelial lining of the blood vessels themselves, or 

 from blood-islands which form on the ventral side of the yolk mass, or 

 they may possibly form from both these sources. 



Fig. 346. 



The development of the posterior part of the venous system in the 

 frog. A. Portion of a transverse section through the posterior meso- 

 nephric region of an 18 mm. tadpole. B. Diagram of the veins of a 25-30 

 mm. tadpole. C. Diagram of the veins of the adult frog, a, Dorsal aorta; 

 c, vena cava ; e, nuclei of the endothelial lining of the mesopheric sinus, 

 continuous with the vascular endothelium ; /, femoral vein ; i, iliac vein ; Ic, 

 lateral mesonephric channel of the posterior cardinal vein ; m, mesentery, 

 mn, mesonephros ; n, mesonephric tubules ; p, posterior cardinal veins (in 

 C showing their original location ) ; pv, pelvic vein ; rp, renal-portal vein ; 

 rr, revehent renal veins ; sc, sciatic vein ; st, nephrostome ; u, caudal vein ; 

 vcm, median mesonephric channel of the posterior cardinal vein ; W, 

 Wolffian duct ; x connection between caudal vein and the lateral meso- 

 nephric channels ; 1-1, part of the renal-portal vein formed from the 'lateral 

 channel of the posterior cardinal ; 2-2, part of the renal-portal vein formed 

 from the median channels of the posterior cardinal vein. (After Shore.) 



THE VENOUS SYSTEM (Figs. 308, 346) 



Omphalomesenteric, Vitello-intestinal, or Vitelline Veins. These 

 veins are really the first part of the vascular system to form in the re- 



