1909. | SUBDIVISIONS OF THE BODY-CAVITY IN BIRDS. 215 
at their extremities, projecting from the dorsal walls of the pleural 
parts of the coelomic cavity. Posteriorly they unite with the 
pericardio-pleuro-peritoneal membrane, and thus for a short space, 
the pleural and peritoneal cavities are separated from one 
another, ‘This is shown in text-fig. 26, which is of a transverse 
section of a chick of 7 days’ incubation. On the left side of 
the section this connection has disappeared ; on the right side it 
is lost a few sections further back, and then the two cavities 
communicate freely. Here the pronephric folds hang free 
from the lateral body-walls into the ccelom—as shewn on the 
right side of the section—until they unite with the Wolffian 
bodies. 
Text-fig. 27 is of a vertical longitudinal section through a 
chick of the same age. 
On the eighth day of incubation, immediately above their 
connection with the pericardio-pleuro-peritoneal membrane, the 
ventral pulmonary ligaments have united with the lateral body- 
walls, and thus formed a part of the post-pulmonary septum. 
The air-sacs are now quite large, and by their backward growth 
have pushed the connective tissue of the more median portions of 
the ventral pulmonary ligaments, together with the tissue of the 
pericardio-pleuro-peritoneal membrane at the posterior edge of the 
lungs—which at this time cannot be distinguished from the 
pulmonary tissue itself—before them into the peritoneal cavity. 
This post-pulmonary tissue on either side, containing the air- 
sacs, forms the antero-lateral wall of the pulmo-hepatic recess. 
Text-fig. 28, p. 216, is a reconstruction similar to text-fig. 25 of 
a chick 8 days after the commencement of incubation. The pleural 
cavities are seen closed ventro-posteriorly by the union of the 
ventral pulmonary ligaments with the lateral body-walls. More 
dorsally, however, the communications with the peritoneal cavity 
are widely open, as is indicated by arrows. 
Ventrally, the pronephric folds are attached to the dorsal 
surface of the ventral pulmonary ligaments where these unite with 
the lateral body-walls; behind this connection the folds hang 
free into the celom, bearing the Miillerian ducts at their 
extremities, until on the dorsal surface they unite with the 
Wolttian bodies. 
On the tenth day of incubation, the pleural and peritoneal 
cavities are completely separated by the dorsal extension of the 
union between the ventral pulmonary ligaments and the lateral 
body-walls on either side. Besides this, however, the backwardly 
growing post-pulmonary tissue containing the air-saecs, which 
appeared on the eighth day as projecting freely into the peritoneal 
cavity, has become united with the lateral body-walls behind 
the ventral pulmonary ligament connection. As this union takes 
place gradually during the ninth day, from before backwards, the 
anterior prolongations of the peritoneal cavity—previously 
forming the lateral boundaries of the post-pulmonary masses of 
tissue—become completely obliterated. Thus the post-pulmonary 
septum, Bertelli’s ‘‘ diaframma primario,” is established. 
