104 



THE EARLY DEVELOPMENT OF MAMMALS. 



umbilical veins, which are differentiated a little later from the vascular plexus of 

 the somatopleure. (Compare Figs. 90, 93, and the description of the veins in the 

 human embryo of the ninth stage, p. 143.) 



The pulmonary veins represent a new system of vessels added to those already 

 mentioned. Their exact origin has not been traced, but about the time the branch- 

 ing of the entodermal lung begins, they appear and open into the left auricle of 

 the heart. 



The eight pairs of primitive veins pass through complicated metamorphoses 

 owing principally, first, to the development of new branches to receive the blood 

 from the organs as they arise; second, to the conversion of small collateral channels 



Ao.ht. 



Ve.ht. 



Om.mes. 



FIG. 59. CHICK OF ABOUT 40 HOURS. VIEW PROM UNDERNEATH OF PART OF THE VASCULAR SYSTEM. 



Ao.ht, Aortic limb of endothelial heart. Om.mes, Omphalo-mesaraic vein. PI, Part of the vascular plexus of the 



area vasculosa. Ve.ht, Venous end of the endothelial heart. X 25 diams. 



into main vessels; and, third, to the obliteration of parts of the original vessels. 

 The first process is illustrated by the history of the veins of the limbs; the second 

 by the formation of the lateral vein of the head and of the subcardinal vein of 

 the Wolffian body; the third, by the disappearance of the right umbilical vein and 

 of portions of the omphalo-mesaraic veins. 



The vena cava inferior is a new pathway formed by utilizing portions of several 

 originally distinct and separated venous channels. At first the blood from the ab- 

 dominal viscera must return to the heart chiefly through the posterior cardinal 

 veins (Fig. 53, card), but the vena cava inferior offers a direct course. Its 

 development depends primarily upon the union of the cephalad end of the right 

 Wolffian body with the liver, followed by a vascular fusion of the two organs which 

 renders it possible for the blood of the right subcardinal vein to pass through the 



