342 THE DEVELOPMENT OF THE CHICK 



I find it in the chick as a very minute vestige at the cranial end of the 

 mesonephros associated with the funnel of the Miillerian' duct. It aids 

 in the final closure of the pleural cavity by bridging over the narrow 

 chink between the lateral angle of the pleuro-peritoneal membrane and 

 the lateral body-wall. (See BerteUi, 1S98.) 



The oblique sei^tum of birds arises as a layer split off from 

 the septum pleuro-peritoneale (pulmonary aponeurosis or pul- 

 monary diaphragm of adult anatomy) by the expansion of the 

 anterior and posterior thoracic air-sacs within it. This mode 

 of formation is clearly seen, particularly on the right side, in a 

 series of transverse sections of a chick embrvo of eleven days 

 (Fig. 190). Thus the cavity bet\veen the oblique septum and the 

 pulmonary diaphragm (cavum sub-pulmonale of Huxley) is not 

 a portion of the body-cavity and bears no relation to it. The 

 ingrowth of muscles into the pulmonary diaphragm can be 

 observed in the same series of sections. It begins on the tenth 

 day according to Bertelli. 



III. The Mesenteries 



The dorsal mesentery is originally a vertical membrane 

 formed by reduplication of the peritoneum from the mid-dorsal 

 line of the body-cavity to the intestine; mesenchyme is contained 

 from the outset between its peritoneal layers, and serves as the 

 pathway for the development of the nerves and blood-vessels 

 of the intestine. In the course of development, its lower edge 

 elongates with the growth of the intestine, and is throwm into 

 folds, or twisted and turned with the various folds and turnings 

 of the intestine. Detailed studies of its later development in the 

 chick have not been published, but the principal events in its 

 history are as follows: For convenience of description the dorsal 

 mesentery may be divided into three portions corresponding to 

 the main divisions of the alimentary tract, viz., an anterior 

 division belonging to the stomach and duodenum, sometimes 

 known as the mesogastrium; an intestinal division belonging to 

 the second loop of the embryonic intestine that descends into 

 the umbilicus; and a posterior division belonging to the large 

 intestine and rectum. Inasmuch as the duodeno-jejunal flexure 

 (Figs. 179 and 180, X) retains from an early stage a short 

 mesenterial attachment, there is quite a sharp boundary in the 

 chick between the first and second divisions of the dorsal 



