THE BODY-CAVITIES 



335 



the heart and liver, (b) a median portion comprising the sinus 

 venosus, ductus venosus and liver, and (c) 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 



jr. 



FIG. 192. Reconstruction of the septum transversum and 

 associated mesenteries of a chick embryo of 80 hours. (After 

 Ravn.) 

 Ao., Aorta. Int., Intestine. Liv., Liver. PL m'g., Plica 



mesogastrica. S.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 ligament appears 

 as a separate membrane (inferior part of . the primary ventral 



