1909. ] SUBDIVISIONS OF THE BODY-CAVITY IN BIRDS. 211 
the pericardium, Butler takes the pulmonary aponeurosis and 
the oblique septum as forming together a single septum, which he 
terms the avian diaphragm. The backward prolongation of this 
in connection with the development of the posterior air-sacs, he 
calls the oblique abdominal septum. Since, however, Bertelli’s 
diaframma ornitico is but half of Butler’s avian diaphragm, I shall 
use neither of these terms, but call the whole of Butler’s avian 
diaphragm together with his oblique abdominal septum, the post- 
pulmonary septum ; this, in the early stages of its development, 
corresponds to what Bertelli calls the “ diaframma primario.” 
For the air-sacs I shall use the terms employed by Bertelli in 
his earlier paper (7]:—Anterior intermediate, posterior inter- 
mediate, and posterior; except for the latter, which he calls 
abdominal, Butler uses the same names. 
The blind anterior diverticula of the peritoneal cavity I shall 
call, after Butler, the pulmo-hepatic recesses; these are the 
“‘ yecessi polmonali-enterici ” of Bertelli. Butler’s pulmo-hepatic 
ligaments correspond to Bertelli’s ventral pulmonary ligaments, 
and I shall use these latter terms in describing the development. 
Finally, I shall call, with Butler, the horizontal septum of 
Beddard [1] the post-hepatic septum; since this term, while 
indicating its essential position, does not depend upon its direction 
which is very variable. 
I. Empryowocy. 
The Post-pulmonary Septum. 
As it is important to follow the growth of the lungs from their 
first appearance, Bertelli begins his observations on chicks of 60 
hours’ incubation, though the septum does not begin to develop 
until much later. At this stage the bronchi are not yet 
formed, but the mesoderm into which they will grow appears 
as slight folds (meso-laterali) on the lateral faces of the median 
dorsal mesentery (setto mesenterico), projecting into the cceelomic 
cavity. That on the right side is united ventrally with the 
septum transversum, this being the vertical partition posterior 
to the pericardium which divides the latter from the peritoneal 
cavity, and in its more dorsal portion carries the two ductus 
Cuvieri inwards to the heart. After three days’ incubation, the 
folds have increased in size, and both are now united ventrally 
with the septum transversum. The bronchi are formed, and 
have penetrated into the mesodermal folds, and so divided the 
latter each into three regions: a posterior, dorsal pulmonary 
ligament, a median, pulmonary rudiment (abbozzo polmonale), 
and an anterior, ventral pulmonary ligament. Between the 
ventral pulmonary ligaments on each side, and the median dorsal 
mesentery, lie the narrowed anterior prolongations of the peri- 
toneal cavity—the pulmo-hepatic recesses. On the fourth day, the 
relations between these parts remain the same, but in a chick of 
five days’ incubation the pulmonary rudiments have greatly 
