THE BODY-CAVITIES 330 
the heart and liver, (b) a median portion comprising the sinus 
venosus, ductus venosus and liver, and (¢) an inferior portion. 
The superior part persists in the region of the sinus venosus and 
liver, and the inferior part only as the primary ventral ligament 
of the liver. 
The median mass of the septum transversum thus includes 
the sinus venosus, liver, and dorsal and ventral ligaments of the 
liver. 
At sixty hours the median mass includes chiefly the sinus 
and ductus venosus and their mesenteries. At eighty hours 
(Fig. 192) a constriction begins to appear between sinus and 
Fia. 192. — Reconstruction of the septum transversum and 
associated mesenteries of a chick embryo of 8O hours. (After 
Ravn.) 
Ao., Aorta. Int., Intestine. Liv., Liver. Pl. m’g., Plica 
mesogastrica. §.V., Sinus venosus. 
ductus venosus, and the walls of the latter are expanded by the 
formation of liver tissue, so that the cylindrical form charac- 
teristic of sixty hours is lost, and the lateral walls of the ductus 
venosus bulge considerably. The continued growth of the liver 
causes a rapid lateral expansion of this portion of the septum 
transversum (Fig. 193 A). 
The primary ventral ligament of the liver is included within 
the wall of the anterior intestinal portal up to about eighty hours. 
But, as the yolk-sac shifts farther back, this hgament appears 
as a separate membrane (inferior part of the primary ventral 
