124. 



THE THIRD DAY. 



Fig. 39. B. 



[CHAP. 



Diagram of the Venous Circulation on the Third Day. 



II. Heart. D.C. Ductus Cuvieri. S. V. Meatus venosus. Sit. V. Superior 

 vertebral or anterior cardinal vein. C. Inferior or posterior cardinal vein. 

 Of. Omphalo-mesaraic vein. 



laries in which these end are gathered into veins which 

 unite to form two main trunks on either side, the cardinal 

 veins, anterior and posterior (Fig. 23, Fig. 39 B, Su. V. and C), 

 which run parallel to the long axis of the body in the upper 

 part of the mesoblast, a little external to the protovertebrse. 

 These veins, which have not by the third day attained to any 

 great importance, unite opposite to the heart, on each side, 

 into a short common trunk at riffht angles to themselves. 

 The two short trunks thus formed, which bear the name of 

 Ductus Cuvieri (Fig. 23, Fig. 39 B, B.C.), running trans- 

 versely straight inwards towards the middle line fall into the 

 sinus venosus. 



The blood-vessels in the body of the embryo take their origin from the 

 mesoblast exclusively, and their formation is probably precisely similar to that of 

 the vessels in the vascular area, branches being £iven out from or brought into 

 connection with the aorta and omphalo-mesaraic veins, in the same way as 

 branches were described as springing from or meeting the earliest formed vas- 

 cular channels. 



His, carrying ont the views we have already referred to, believed that 

 parablastic elements i_ r rew inwards along the omphalo-mesaraic trunks, through 

 the length of the heart and so onwards into the aorta and all its branches, until 



