1909. ] SUBDIVISIONS OF THE BODY-CAVITY IN BIRDS. 217 
of these structures in a chick of 10 days’ incubation. In addition, 
this figure shows the origin of the first costo-pulmonary muscles 
from the fifth ribs, which run into the post-pulmonary septum 
following the ventral and posterior border of the lungs. These 
will be supplemented by others arising later from the third, 
fourth, and sixth ribs, and give rise to the aponeurotic mem- 
brane, which in the adult covers the postero-ventral face of the 
lungs and separates these organs from the intermediate air-sacs. 
The pronephric folds are attached ventrally to the antero- 
lateral walls of the post-pulmonary septum, and dorsally are con- 
nected with the external margins of the Wolffian bodies. 
Text-fig. 29. 
Reconstruction, similar to text-figs. 25 and 28, of a chick of 10 days’ ineubation. 
The post-pulmonary septum, now fully formed from the median 
dorsal mesentery in the middle line, the pericardio-pleuro- 
eritoneal membrane with the ventral pulmonary ligaments, and 
slightly also from the pronephric folds laterally, constitutes a 
complete partition dividing the celomic cavity into a pleural 
and a peritoneal portion. JBertelli calls this septum the 
Proc. Zoot. Soc.—1909, No. XV. 15 
