MORPHOGENESIS OF CIRCULATORY SYSTEM 73 



(Chap. 13). During this process, the vitelline veins are fenestrated, an( 

 sinusoids are produced. These sinusoids connect with the right and left vitellin 

 (hepatic) veins at the anterior end of the liver. 



Posterior to the liver, the right and left vitelline veins form a collar aroun( 

 the duodenum as shown in figure 334C. The left portion of the duodena 

 collar then disappears, and the hepatic portal vein which receives blood fron 

 the developing stomach, pancreas, and intestine enters the liver as indicate! 

 in figure 334D. 



As the above development progresses, three important changes are effectet 

 (fig. 334E): 



( I ) The lateral veins along the lateral body wall arise and join the commoi 

 cardinal veins near the entrance of the right and left vitelline (hepatic 

 veins; 



(2) the intestinal vein loses its connection with the caudal vein; and 



(3) the postcardinal veins extend caudally and ct>nnect with the cauda 

 vein. 



Meanwhile, the mesonephric kidneys begin to develop, and new veins, ii 

 the form of irregular venous spaces, form between the two kidneys. Thesi 

 new veins are the subcardinal veins. The subcardinal veins are joined by th^ 

 internal renal veins which ramify through the kidney substance from the pos 

 terior cardinal veins. They course over and around the forming renal tubule 

 (fig. 334F, G). 



Later, the two subcardinal veins extend forward and by means of an anas 

 tomosis on either side connect with the posterior cardinal veins anterior t( 

 the mesonephric kidneys. As this transformation occurs, the segment of eacl 

 posterior cardinal vein atrophies between the kidney and the point where th^ 

 subcardinal venous anastomosis joins the posterior cardinal vein (fig. 334G) 



While the above changes evolve, the anterior cardinal veins expand greatl; 

 over the dorsal pharyngeal area, where they form sinus-like spaces. Thesi 

 anterior cardinal venous sinuses receive the internal jugular veins from th 

 brain region and various pharyngeal veins. Coronary veins and externa 

 jugular veins also develop as shown in figure 334H. 



b. Changes in the Primitive Converging Veins of the Heart in the 

 A nuran A mphihia 



1) Vitelline Veins. As in the shark embryo and in all other vertebrates, thi 

 vitelline veins of the frog or toad embryo are among the first blood vessel 

 to be formed in the body. In frog embryos of about 3- to 4-mm. in length 

 the two vitelline veins begin to appear as irregular blood spaces along thi 

 ventro-lateral aspect of the midgut region, extending anteriad around thi 

 forming liver. At a point immediately anterior to the liver rudiment, thesi 

 vessels fuse to form the endocardinal rudiment of the heart (fig. 332I-K) 



