470 Description of a 4-mm. Human Embryo 
body wall forward, at a level ventral to the intestine, varying only slightly 
in diameter. At points differing slightly on the two sides, more an- 
teriorly and dorsally on the left, they join the two jugular veins and 
turn inward (on the left side also ventrally) as the sinus venosus. From 
each jugular vein, nearly at its junction with the umbilical vein, arises 
a small bud, the future posterior cardinal vein (P. card.). The Duct of 
Cuvier (D. C.), later a well marked trunk extending from the junction 
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P. card. 
Fic. 12. Model of veins near heart, from left side. X 160. 
of jugular and cardinal veins to the entrance of the umbilical veins, can 
in this embryo, then, scarcely be said to exist, so short and ill-defined 
is it. 
The sinus venosus is seen, as in the model of the heart, extending 
across the embryo, an irregular cavity subdivided by bands of mesen- 
chyma (represented in this cast as holes), and emptying anteriorly into 
the right auricle by two channels (Rt. au.). Into this sinus empty also 
the two vitelline veins (Vit. v.) one on each side of the intestine, at a 
level dorsal to the umbilical and jugular veins (Fig. 12). The vitelline 
veins, unlike the two other pairs modelled, vary greatly in caliber; they 
are small and rounded at their entrance into the sinus venosus, but more 
caudally spread into large channels, each with a mesial wall straight, 
