212 MISS M, POOLE ON THE DEVELOPMENT OF THE  [ Mar. 2, 
increased in size, and the ventral pulmonary ligaments have 
united ventrally with the lower edge of the median dorsal mes- 
entery, the dorsal surface of the liver, and the pericardio-pleuro- 
peritoneal membrane. The latter is what is called in younger 
stages the septum transversum ; it now forms a mass of tissue 
separating the pericardium from the pleural and the peritoneal 
regions of the celom.  Posteriorly, the ventral pulmonary 
ligaments spread out laterally to unite with the antero-lateral 
corners of the now well-developed liver; and at the same time the 
dorsal pulmonary ligaments and the lung rudiments themselves 
assume an almost transversely horizontal position; so that in this 
region there is now a septum across the ccelomic cavity, complete 
except at its extreme lateral margins where the pleural and 
peritoneal divisions communicate by a narrow aperture on either 
side. Ata later stage these passages will be closed by the union 
of the ventral pulmonary ligaments and the lateral body-walls, 
and thus the post-pulmonary septum—bBertelli’s ‘“ diaframma 
primario ’—will be formed. 
The pronephric folds appear at this stage as thickenings of the 
connective tissue on the external walls of the anterior part of the 
pleuro-peritoneal cavity; posteriorly they are seen in close con- 
nection with the developing Wolffian bodies on the dorsal wall of 
the ccelomic cavity. 
Text-fig. 24 is of a transverse section of a chick of 5 days’ 
Text-fig. 24. 
Transverse section of a chick of 5 days’ incubation, through the anterior region 
of the liver; anterior face. 
For explanation of abbreviations in text-figures see p. 235. 
incubation, taken through the region where the septum will be 
formed, the anterior face being shown. The developing pro- 
nephros is seen on the dorsal wall of the pleuro-peritoneal cavity. 
