220 MISS M. POOLE ON THE DEVELOPMENT OF THE  [ Mar. 2, 
The post-pulmonary septum stretches across the coelomic cavity 
from one side to the other, closely fused to the lateral body-walls 
throughout its entire thickness. In it, near the middle line, lie 
the anterior intermediate air-sacs. The costo-pulmonary muscles 
pass inwards from the ribs dorsal to the air-sacs following the 
ventral border of the lungs. They really form a continuous 
layer dorsal to the cesophagus, but in the section figured they are 
interrupted by the ostia of the anterior intermediate air-sacs. 
Forming the ventral boundaries of the post-pulmonary septum 
are, medianly, the pulmo-hepatic recesses, and laterally, the 
abdominal cavity. Behind the pericardium at this stage the 
posterior intermediate air-sacs, still comparatively small, lie in the 
periphery of the septum, and more posteriorly the posterior air- 
sacs occupy the same position. These however have extended 
both ventrally and also towards the middle line, always carrying 
some tissue of the post-pulmonary septum with them, and have 
united with the median mesentery of the gizzard and liver, dorsal 
to these organs and ventral to the Wolffian bodies. This union 
forms the beginning of the post-hepatic septum to be described 
below. Text-fig. 31 is of a transverse section of the same chick of 
13 days’ incubation, taken through the region of the spleen, 
and showing the inwardly expanded post-pulmonary septum— 
containing the greatly enlarged posterior air-sacs—united to the 
gastro-hepatic ligament. 
The Post-hepatic Septun. 
This septum is completed in the chick on the 15th or 16th day 
of incubation. The first to be formed is the antero-dorsal moiety, 
and this is fully developed by the 15th day. Text-fig. 31, as I 
have already described, shows the formation of this part by the 
inward expansion of the two halves of the post-pulmonary septum 
(those parts termed by Butler the ‘oblique abdominal septa’) 
towards the middle line, where they meet and fuse with the 
mesentery of the gizzard and liver. This lateral extension is 
due to the greatly increased size of the posterior air-sacs enclosed 
within the septum. 
Ventral to this connection the gizzard is seen in text-fig. 31 
included within the median mesentery, which is therefore split 
into two layers. Posteriorly this mesentery expands laterally 
before it becomes attached to the ventral body-wall; and at a 
later stage its lateral edges become fused to the side walls of the 
peritoneal cavity, thus constituting the postero-ventral portion of 
the post-hepatic septum. When fully formed, this is a membrane 
passing obliquely from the antero-dorsal to the postero-ventral 
walls of the peritoneal cavity and enclosing within its thickness the 
gizzard in the more posterior portion. It is a complete partition 
except on the left side where the dorsal and ventral components 
never quite meet, and so there remains always a narrow com- 
munication between the post-hepatic intestinal cavity and the 
