THE BODY-CAVITIES 341 
from which it arises (plica mesogastrica, Broman) can be traced 
back, shifting its attachment to the dorsal mesentery, as far as 
the anterior intestinal portal and a little farther (Fig. 192, ef. 
also Fig. 120). 
At ninety-six hours the entodermal lung-saes extend far into 
the accessory mesenteries, and thus lie laterally to the pneumato- 
enteric recesses. On the left side the accessory mesentery ceases 
opposite the tip of the lung, but on the right side it is continued 
back by the mesentery of the vena cava as far as the middle of 
the stomach, and in this region its ventral attachment is to the 
superior lateral angle of the liver. 
The growth of the lung-sacs into the accessory mesenteries 
divides the latter into three parts, viz., a superior portion uniting 
the lung to the dorsal mesentery, a median portion enclosing the 
lung, and an inferior portion uniting the lung-sacs to the median 
mass of the septum transversum. Now, as the liver expands 
laterally the ventral attachment of the accessory mesentery is 
‘arried out towards the lateral body-wall, inasmuch as its attach- 
ment is to the lateral superior face of the liver (cf. Fig. 231, Chap. 
XII). Thus the accessory mesenteries are gradually shifted 
from their original almost sagittal plane to a plane that is approxi- 
mately frontal. The developing lungs project dorsally from the 
accessory mesenteries, which may now be called the pleuro- 
peritoneal membranes, into the pleural cavities (Fig. 189); and 
the latter communicate with the peritoneal cavity only laterally 
to the liver. These communications are then soon closed by a 
fusion between the lateral edges of the pleuro-peritoneal mem- 
brane and the lateral body-wall; this fusion is not completely 
established on the eighth day, but it is on the eleventh day. 
In reptiles and mammals the so-called mesonephric mesentery plays 
an important part in the closure of the pleural cavities. It arises from 
the apex of the mesonephros at its cephalic end, and fuses with the septum 
transversum. It thus forms a partition between the hinder portion 
of the pleural cavity and the cranio-lateral recesses of the peritoneal 
cavity. Subsequently, in mammals, its posterior free border fuses with 
the caudal bounding folds of the pleural cavity that arise as forwardly 
directed projections from the accessory mesentery on the right side 
and the wall of the stomach on the left. Hochstetter states that such 
a mesonephric fold is found in the chick but that it does not appear to 
play any essential part in the formation of the septum pleuro-peritoneale. 
