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 Mullerian 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 Bertelli, 1898.) 
The oblique septum 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 embryo of eleven days 
(Fig. 190). Thus the cavity between 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. Tar MrsentTerius 
The dorsal mesentery is originally a vertical membrane 
formed by redupliecation 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 thrown 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 
(igs. 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 
