1909. | SUBDIVISIONS OF THE BODY-CAVITY IN BIRDS. 233 
Penguins, that the muscle referred to by Huxley in the Crocodile 
is the equivalent of this muscle, for in both cases the fibres arise 
from the pubis. Beddard therefore considers it more justifiable 
to identify the middle portion of the horizontal septum of the 
Crocodile with that structure in birds, while the lateral portions 
containing the muscles he compares with the oblique septa of 
birds. And this comparison he considers is made more obvious 
by the relations of the two septa in Dromeus; for here the oblique 
septa are not attached to the ventral body-wall, but posteriorly 
project with a free edge into the visceral cavity, appearing as a 
fold of the horizontal septum. This peculiarity in the Emu 
Beddard takes to support his contention that “the omentum 
(horizontal septum) as well as the oblique septa of birds are to be 
derived from the fibrous expansion which covers over the viscera 
in the Crocodilia.” He also suggests that possibly “the oblique 
septum of birds has been produced by a vertical fold of this ex- 
pansion.” This does not seem to me likely however, as in 
development it is the oblique septa which are first formed, and 
only much later does the horizontal septum arise, partly as folds 
of the former. Also, against the above comparison of the lateral 
parts of the crocodilian horizontal septum with the oblique septa 
of birds, should I think be set the fact, that the only cause of 
the posterior attachment of the oblique septa in the latter being 
so far back as to bear a resemblance to the position of the lateral 
parts of the horizontal septum in Crocodiles, is the growth of the 
posterior air-sacs. But since these structures are not present in 
reptiles, we should expect to find the post-pulmonary septum but 
little extended, and comparable more or less to the embryonic 
condition in a bird (¢f. text-fig. 25, p. 213) before the air-sacs are 
developed. And this is what we do find, if we take only that tissue 
on the ventral face of the lungs in the Crocodile as representing 
the avian post-pulmonary septum. 
As to the origin and phylogenetic significance of the subdivisions 
of the pleuro-peritoneal cavity, it is impossible at present to say 
anything definite; for while on the whole the condition in the 
Crocodilia seems to approach most nearly that in Birds, yet the 
manner of the exclusion of the lungs from the peritoneal cavity 
in the latter is far more like that which occurs in TVestudo and 
probably also in the Varanide. While again the post-hepatic 
septum—not represented in these two forms—is found, in the 
same way as in the Crocodile, in the Telide, where it appears 
perfectly comparable to the more ventral portion of the avian 
post-hepatic septum; and according to Butler, it is also com- 
pletely represented in Snakes. Therefore, whether or not the 
subdivision of the celom will prove of importance as to the 
phylogeny of the Reptilia and Aves, must, I think, remain 
undecided until further embryological work has been done. 
Proc. Zoou. Soc.-——1909, No. XVI. iG 
