A. M. Miller 289 
lived, for in an embryo but slightly older than that represented in 
Fig. 5 the postcava joins the 
ductus venosus. ‘The postcava 
after it makes its exit from the 
caval mesentery joins the right 
subcardinal some distance in 
front of the origin of the A. 
omphalomesenterica. Thus the 
posterior portion of the right 
subeardinal becomes the direct 
continuation of the postcava, 
and a renal portal system is 
established on the right side. 
Soon after this, however, an 
anastomosis takes place be- 
tween the two subcardinals 
ventral to the aorta and caudal 
to the A. omphalomesenterica, 
and by this means the blood 
from the left subcardinal also 
passes into the postcava. This 
brings about a complete renal 
portal system. The anasto- 
mosis between the two subcar- 
dinals marks the future bifur- 
cation of the adult postcava. 
As the mesonephroi enlarge 
and occupy a considerable por- 
tion of the body cavity they 
grow around the subcardinals, 
which thus become enclosed 
within their median side and 
are carried somewhat ventrad. 
This brings about a closer rela- 
tion between the mesonephroi 
and the subcardinal veins. 
In consequence of this closer 
relation the latter vessels in- 
crease in size and receive blood 
from the mesonephroi through 
Fig. 5. Reconstruction of the venous system of 
a chick of 90hours. Ventral view. 50. (Hxpla- 
nation of lettering on page 284.) 
large branches, as indicated in Fig. 6 (a) (chick of 5 days’ incubation). 
With the increased amount of blood from the mesonephroi and the en- 
