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 
Dic:s: 
V.us 
Fig. 211. — 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., Duetus (meatus) venosus. 5S. 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 postecardinals (Fig. 212), replaced after- 
wards (fifth day) by a renal portal circulation through the sub- 
stance of the mesonephros. As the subeardinal 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 subeardinal (Fig. 213). 
The venous circulation is then as follows: The blood from 
