L E E I DOS I R E X PA R A DO X A . 
113 
A little later (Stage 24) the inner ventral ends of these 
endothelial vessels fuse and form the posterior part of the 
primitive heart-tube from which the sinus venosus is deve- 
loped (Text-fig. 24 B, V. V .) . 
The peripheral ends of the vitelline veins communicate 
with the blood-spaces appearing on the lateral surfaces of 
the yolk (Stages 24-25), and simultaneously the anterior 
cardinal veins appear in the lateral regions of the head, 
passing backwards dorsal to the developing branchial vessels 
as far as the inner margins of the anterior ends of the deve- 
loping pronephric sinuses (posterior cardinal veins) on either 
side. The pronephric sinuses in turn are, as in Poly- 
p ter us ( 15 ), connected for a time at intervals along their 
outer margins with the vitelline meshwork; and also as in 
Polyp ter us and Belone (22) the most anterior of these 
connections curves outwards, downwards and forwards, 
communicating directly with the vitelline vein going to the 
heart and thus forming a venous arch over the lateral 
surface of the yolk. These venous arches, ducts of Cuvier, 
each receive for a time a main lateral vitelline vein from the 
lateral surface of the yolk, both of which vessels become 
modified later on with the development of the liver and sub- 
intestinal vein (Text-fig. 24 c, r.D.C. and I.D.C.). As the 
yolk is absorbed the ducts of Cuvier finally become much 
shortened, forming straight lateral trunks on either side of 
the heart, and giving rise to the sinus venosus at their point 
of junction behind it. With the appearance of blood-spaces 
in the liver (Stages 30-31) the ducts of Cuvier become much 
of developing cardinal veins. E. Diagram of venous system. 
A. Card. Anterior cardinal. Cau. V. Caudal vein. Ii. Heart 
II. V. Iliac vein. Inf.J. Inferior jugular vein. I.R.I. Inter-renal 
vein. L. C. Lateral cephalic section of anterior cardinal. Lat. 
C. Lateral cutaneous vein. I. D. C. Left ductus Cuvieri. 
?. P. Curd. Left j)osterior cardinal. M. C. Obliterating median 
cephalic section of anterior cardinal vein. P. Card. Posterior 
cardinal vein. P. C. Posterior cerelDral. P. V. C. Posterior 
vena cava. P. P. Rectal vein. P. P. Renal portal vein. r.P. 
Card. Right posterior cardinal vein. r. D. C. Right ductus 
Cuvieri. r. 1. V. V. Right lateral vitelline vein. Scl. Sub- 
clavian. Si.V. Subintestinal vein. S. P. Sinus venosus. I. I. 
A^itelline veins. 
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