340 THE DEVELOPMENT OF THE CHICK 



this subsequently becomes incorporated in the right auricle of 

 the heart. (See Chap. XII). 



II. SEPARATION OF PLEURAL AND PERITONEAL CAVITIES; ORIGIN 

 OF THE SEPTUM PLEURO-PERITONEALE 



The pleuro-peritoneal septum arises from the so-called acces- 

 sory mesenteries, the origin of which must now be described. 

 At first the septum transversum has only a median dorsal mesen- 

 tery, viz., the superior part of the primary ventral mesentery 

 that unites the septum transversum to the floor of the fore-gut, 

 and so by way of the dorsal mesentery of the latter to the dorsal 

 body-wall. Subsequently, however, there arises a pair of mesen- 

 teries extending from the lateral wall of the oesophagus to the 

 septum transversum. These are the accessory mesenteries, and 

 they arise as follows: about the sixtieth hour they appear as 

 mesenchymatous outgrowths, forming elongated lobes, projecting 

 from the side walls of the oesophagus opposite the hind end of 

 the lung rudiments. The right and left lobes are practically 

 the same size at first and they bend over ventrally and soon fuse 

 with the median mass of the septum transversum, represented 

 at this time by the sinus and meatus venosus (cf. Figs. 118-120, 

 Chap. VI). Thus are produced a pair of bays of the peritoneal 

 cavity ending blindly in front, bounded laterally by the accessory 

 mesenteries, and in the median direction by the intestine and 

 its mesenteries. These are the pneumato-enteric recesses. 



These bays have received different names from the various authors : 

 thus His named only the right one as recessus superior sacci omenti; 

 the left one being practically absent in mammals; Stoss called both re- 

 cessus pleuro-peritoneales ; Mall called them gastric diverticula; Hoch- 

 stetter, recessus pulmo-hepatici ; Maurer, bursa hepatico-enterica ; Ravn, 

 recessus superior for the right one and recessus sinister for the left. We 

 may call them the pneumato-enteric recesses (recessus pneumato-enterici) , 

 following Broman. 



At seventy-two hours the entodermal lung-sacs extend to 

 the base of the accessory mesenteries, ending at the anterior 

 end of the pneumato-enteric recesses. On the left side at this 

 time the recess is fully formed back to near the anterior end of 

 the cephalic hepatic diverticulum, on the right side considerably 

 farther back; that is, the accessory mesentery is already longer 

 on the right than on the left side, and the mesenchymatous lobe 



