Ti EP I DOSIU EN PA R A DOX A . 
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a little later (Stage 31 -f), passing inwards from the rudi- 
mentary pectoral limbs and entering the anterior part of the 
pronephric sinuses on their outer sides (Text-fig. 24 e, ScL), 
As the pronephric tubules and the outer part of their sinuses 
atrophy, the subclavian veins come to open into the outer 
sides of the proximal part of the anterior cardinal veins 
approximately opposite the entrance to the anterior jugular 
vessels. In the adult, owing partly to the straightening of 
the proximal parts of the anterior cardinal veins, which are 
at first directed laterally, and also partly to the marked 
elongation of the body in Le pi do siren, the inferior jugular 
and subclavian veins appear to enter comparatively far 
forwards along the anterior cardinal vessels (Text-fig. 3, Inf. 
J. and ScL). At Stage 32 two small lateral cutaneous veins 
appear one on either side, opening into the subclavian veins 
from behind (Text-fig. 24 e, Lat. C .) . They pass a little 
distance backwards in the superficial layers of the body-wall 
and receive two or three small veins through the body 
musculature from the vertebral region. 
Coronary Yein. — The coronary vein appears on the 
right outer wall of the ventricle while that surface of the 
heart is still applied against the sinus venosus posteriorly 
(Stages 31-32) — that is to say, while the heart loop is still 
transverse to the axis of the embryo. The little vessel opens 
from the surface of the ventricle into the sinus, and as the 
long axis of the ventricle becomes parallel with that of the 
body, the vein is considerably stretched and carried on to 
the posterior surface of the ventricle. With the heart in the 
adult position the coronary vein is on its right dorsal surface, 
and opens on to the floor of the sinus venosus behind a little 
fold whose free margin is directed posteriorly. 
YI. Origin of Yessel- and Blood-Cells. 
Yes s el-cells. — At Stage 23 the lateral mesoderm is 
delaminating over the surface of the yolk. The rather 
columnar cells of the splanchnic mesoderm in the cardiac 
