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 cesophagus 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 cesophagus 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 
‘avity 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 callthem the pneumato-enteric recesses (recessus pneumato-entericl), 
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 
