114 
JANE T. ROBERTSON. 
expanded^ aud each forms a great venous channel extending 
transversely across the upper part of the anterior surface of 
the liver_, into the outer ends of which the anterior and 
posterior cardinal veins open at right angles opposite one 
another on either side (Text-fig. 24 d, r. D. C.). 
The posterior cardinal vein (prouephric sinuses) thus come 
into close relationship with the sinuses in the liver. The 
venous channels developing in the liver become concentrated 
towards the right in the neighbourhood of the sinus venosus 
aud form a wide vessel opening into it ventrally (Stages 
31-32). 
Posterior Cardinal Veins and Caudal Veins. — 
With the development of the prouephric tubules aud archi- 
nephric ducts, blood-spaces appear round them, foreshadow- 
ing the two posterior cardinal veins (Text-fig. 24 c and D, 
P.Card.). These spaces develop rapidly into considerable 
sinuses at the anterior ends of the tubules and communicate 
with the anterior cardinal veins, forming the ducts of Cuvier 
as described. Posteriorly these sinuses are continued along 
each archinephric duct as two vessel-rudiments, one on the 
inner dorsal and one on the outer ventral surface, which are 
•connected round the duct by frequent anastomoses, forming 
the posterior cardinal veins. The posterior cardinal veins 
extend along the archinephric ducts to their terminations on 
the sides of the cloaca (Stage 24), and, pas.sing a little 
ventrally and forwards, are then continuous with the blood- 
spaces appearing on the ventral surface of the yolk, joining 
the precloacal subintestinal vein, which at this stage is still 
extremely short. An anastomosis occurs between the 
posterior termination of the aorta and the posterior cardinal 
veins at Stage 27 (Text-fig. 25, D. Ao.', P. Card.), and the 
lateral trunks formed by the junction of these vessels (i. e. 
the posterior cardinal vein and the bifurcations of the aorta 
and subintestinal vein on either side) join almost imme- 
diately and form the short caudal vein ventral to the post- 
anal gut. 
As the post-anal gut lengthens, the aorta and caudal vein 
