426 MALL. [Vor XH. 
the beginning of the liver, as Fig. 18 shows. A portion of 
this mesodermal tissue has been described by His as the septum 
transversum.! According to His only that portion of the meso- 
dermal tissue is septum transversum which lies between the 
posterior part of the pericardial cavity (Parietelhohle), the wall 
of the intestine, and the point where the veins enter the heart. 
It extends across the body, and has within it the beginning of 
the liver. In transverse section this region is shown in Fig. 
18. Now the pericardial cavity communicates by means of a 
long canal on either side, with the peritoneal cavity, and the 
omphalomesenteric vein hangs into this, attached to a kind of 
mesentery, as Fig. 18 shows. Lower down, near the commu- 
nication (Fig. 19), there is an indication of the beginning of the 
umbilical vein, which unites with the omphalomesenteric vein 
through the membrana reuniens. The two canals which com- 
municate with the extraembryonic coelom are the pleural cav- 
ities, and the membrana reuniens aids to separate them from 
the peritoneal. 
All of the tissues from the diaphragm to the opening of 
the liver duct into the duodenum arise from the septum trans- 
versum and the membrana reuniens ; the stomach from the 
fore-gut, the liver from the liver diverticulum, and the diaphragm 
from the septum transversum and the membrana reuniens. The 
Cuvierian duct must also have arisen in the membrana reuniens, 
in order to pass around the outside of the body cavity to reach 
the cardinal and jugular veins, as pictured by His? for the 
human embryo. 
In the further development of the pleural and _ pericardial 
cavities the Cuvierian veins give us our best landmark, as they 
define the point where the pleural cavity is to be separated 
from the pericardial. And it really seems, as if the greater 
portion of the diaphragm is formed from the portion of the 
septum transversum on the ventral side of the vein and from 
the membrana reuniens, rather than from the portion immedi- 
ately in front of the intestine. In other words, there is a 
1 His: Anat. mensch. Embryonen, I, p. 126. 
2 His: His’s Archiv, 1881, Plate XII, Fig. 9. Also Anat. mensch. Embryonen, 
Plate IX, Figs. 10-12, 14. 
