THE VENOUS SYSTEM. 



103 



which is matched by a similar vein on the opposite side of the head. The 'remain- 

 ing primitive veins and the later secondary veins are all formed in a like manner. 

 The first veins to be formed are the omphalo-mesaraic, which are evolved from 

 the vascular network of the area vasculosa, and extend into the body of the em- 

 bryo, running in the splanchnopleure (Fig. 158, Om.S, Om.D). They approach 

 one another in the median line (Fig. 59), unite, and are prolonged forward to make 

 the endothelial heart. 



FIG. 58. CHICK EMBRYO WITH TWENTY-FIVE SEGMENTS. DRAWN FROM A SPECIMEN WHICH HAD BEEX AUTO- 

 INJECTED WITH INDIA-INK. (After H. M. Evans.) 



There are three other pairs of main primitive veins, all developed entirely within 

 the body of the embryo: i, the anterior cardinals, which drain the head (com- 

 pare Fig. 154, card, and Fig. 155, Ve)\ 2, the posterior cardinals, which drain the 

 body from the tail to the heart and occupy each a characteristic position laterad 

 from the aorta, and dorsad from the splanchnocele (Fig. 158, card, and Fig. 159, 

 card, card.s}; the two cardinal veins unite at the level of the venous end of the 

 heart and form thus on each side a short transverse stem, the common cardinal, 

 which opens into the heart and was. formerly named the ductus Cuvieri; 3, the 



