1909. | SUBDIVISIONS OF THE BODY-CAVILY IN BIRDS. 219 
entirely confirm Butler’s conclusion, that “the avian diaphragm 
is seen to be completed as a single structure, and its separation 
into its two lamine is a secondary detail arising in connection 
with the development of the two pairs of intermediate air-sacs, 
which first penetrate it at a time when no distinct line can be 
drawn between the tissue that goes to form it, and that of the 
developing lung itself.” 
The adult arrangement of these structures is shown in text-fig. 32, 
p. 222, which is of a section of a Rook through the region of the 
liver and gizzard, seen from the ventral side. The post-pulmonary 
tissue is seen united to the lateral body-walls for a very consider- 
able distance on either side, and ventrally and towards the 
middle line is pushed out irregularly into the peritoneal cavity by 
the growth within it of the posterior air-sacs. The latter, 
however, as I have already said, never project beyond the edge of 
the septum in which they he, but always carry a layer of con- 
nective tissue and cceelomic epithelium in front of them. 
Text-fig. 30 is of a transverse section of a chick, 13 days after the 
peginning of incubation, through that part of the post-pulmonary 
Text-fig. 30. 
D.ms. 
1 
' 
1 
I 
t 
1 
Uy 
U 
Phurs: 
Transverse section of a chick of 13 days’ incubation, through the region of the 
pericardium and anterior intermediate air-sacs ; seen from behind. 
septum which encloses the anterior intermediate air-sacs. The 
lungs are seen now lying in completely closed pleural cavities, 
separated from each other in the middle line by the median 
dorsal mesentery. 
iL 
