LATER DEVELOPMENT OF VASCULAR SYSTEM 369 



the liver, the venous islands flow together and establish a venous 

 trunk extending along and within the right dorsal lobe of the 

 liver, and opening anteriorly into the meatus venosus. At first 

 the connection with the meatus venosus lies near the sinus veno- 

 sus, but in later stages is some distance behind the latter. Behind 

 the liver the dorsal attachment of the caval fold is to the ventral 

 surface of the right mesonephros, and at this place the vena cava 

 enters the mesonephros and connects with the subcardinal veins 

 (cf. Fig. 182). 



The latter vessels arise as a series of venous islands on the 

 median surface of the mesonephros and lateral to the aorta on 

 each side. Such disconnected primordia are first evident at 



D.C.S. 

 V.U.S 



Fig. 21L — A drawing of a wax reconstruction of 



the veins in the region of the liver of a sparrow 



embryo. Outline of the liver represented by 



broken lines. Dorsal view. (After Miller.) 



D. C. d., s., Right and left ducts of Cuvier. 

 D. v., Ductus (meatus) venosus. S. V., Sinus 

 venosus. V.c. i., Vena cava inferior. V. u.d.,s., 

 Right and left umbilical veins. 



about the seventieth hour, and soon they run together to form 

 a longitudinal vessel on each side, which has temporary direct 

 connections with the postcardinals (Fig. 212), replaced after- 

 wards (fifth day) by a renal portal circulation through the sub- 

 stance of the mesonephros. As the subcardinal veins enlarge, 

 they approach one another just behind the omphalomesenteric 

 artery beneath the aorta and fuse together (sixth day. Fig. 213). 

 In the meantime, the post-cava has become continuous with the 

 anterior end of the right subcardinal (Fig. 213). 



The venous circulation is then as follows: The blood from 



