THE BODY-CAVITIES 341 



from which it arises (pHca 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, cf. 

 also Fig. 120). 



At ninety-six hours the entodermal lung-sacs 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 

 carried out towards the lateral body-wall, inasmuch as its attach- 

 ment is to the lateral superior face of the liver (cf. Fig. 231, Chap. 

 XIII). 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. 



