226 MISS M, POOLE ON THE DEVELOPMENT OF THE _ [ Mar. 2, 
would depend only on whether, as in the Rook, the diverticulum 
remains comparatively slightly expanded laterally, and so only 
detaches the peritoneum from the body-wall in the middle line; 
or whether, as may be the case in other birds, it extends far 
round against the lateral body-walls into the post-pulmonary 
septa. 
As I have before mentioned, I cannot agree with Bertelli’s 
contention that the oblique septum, the « diaframma toraco- 
adeomumells,” is not a septum, or part of a septum, at all, but 
‘non é altro che la parete ventrale dei sacchi aeriferi intermedii e 
la parete posteriore del pericardio.” He shows how in the Fowl 
the ventral walls of the air-sacs are covered by muscular tissue, 
and how in the Duck this is more markedly the case; but holds 
that the muscular tissue has nothing te do with the subdivision of 
the body-cavity, but belongs to the walls of the air-sacs, together 
with the nerves which, according to Beddard, are of spinal 
origin. Since, as I have show. the air-sacs never project 
beyond the edge of the post-pulmonary septum, the muscles would 
lie in the connective tissue between the peritoneum bounding the 
abdominal cavity and the endoderm of the air-sacs, and therefore 
in the tissue of the post-pulmonary septum. And I believe that 
the presence of muscular tissue justifies the description of this 
part of the abdominal wall as the postero-ventral portion of the 
post-pulmonary septum. 
Huxley [12] also describes a layer of muscular fibres in the 
oblique septum of the Duck, and Beddard [2] finds a considerable 
sheet of muscle in part of the septum of the Puffin (/ratercula 
arctica). He says: “ This layer of muscular fibres arises from the 
pubis—from the proximal and larger half of this bone; it is 
abundantly furnished with blood-vessels and nerves. The direction 
of the muscle is oblique ; it covers the hinder region of the oblique 
septum, ending abruptly some little way in front of the posterior 
attachment of the latter ; it is attached below to the upper surface 
of the sternum, and to the abdominal parietes along the last 
sternal rib.” Beddard also finds similar muscles in two species of 
Penguins (Hudyptula minor and Spheniscus demersus) and also, 
though feebly developed, in the Toucan (Rhamphastos ariel). 
The Post-hepatic Septum. 
This septum in the adult fowl isa membrane extending, as | 
have shown, obliquely across the visceral cavity from its dorsal to 
its ventral wall. Dorsally and laterally in front it is continuous 
with the post-pulmonary septum, which in turn is united with 
the vertebral column by means of the median dorsal mesentery. 
Posteriorly and postero-laterally it is attached to the ventral 
body-wall. It thus forms a complete septum, except on the left 
side where the pulmo-hepatic recess remains in communication 
with the abdominal cavity by means of a narrow aperture. 
Except for this orifice, the two pulmo-hepatic recesses now form 
