456 MR. G. W. BUTLER ON THE SUBDIVISION OF [Nov. 19, 
No attempt will be made to go over the work that has been done 
on the earlier stages of the chick down to the 6th day of incubation, 
but rather to consider the light thrown on the adult condition by 
the subsequent stages of development *. 
The changes going on, for the most part synchronously, in the 
chick between the sixth and twelfth days may be grouped, so far 
as the subdivision of the body-cavity is concerned, under the 
following heads * :— 
(A) The completion of the avian diaphragm and the develop- 
ment of the ‘‘ diaphragmatic”? and “ abdominal ” air-sacs 
(s.a', s.a", and s.a'"’). 
(B) The development of the oblique abdominal septum (vy), 
partly in connexion with the growth of the ‘abdominal 
alr-sacs.” 
(C) The lateral extension of the ventral ligament of the stomach 
and hinder part of the liver (avian “omentum,” B), so 
that meeting the pulmohepatic ligaments (a) and the 
oblique abdominal septum (y), it on the one hand helps in 
the closing of the ventral liver-sacs (1, 1), and on the 
other in the formation of a post-hepatic septum (B+y) 
(‘‘ diaphragme transversal ”’ of Perrault), which is, however, 
not complete on the left side. 
(D) The assumption of their final form and relations by the 
pulmohepatic recesses (2, 2') and ligaments (a, a). 
III. (a). On the Development of the Air-sacs in relation to the 
parts of the Avian Diaphragm. 
On the 8th day, while the passages between the pleural and 
peritoneal cavities are widely open, the abdominal air-sac first 
1 The following remarks, however, may be added with regard to the 
shutting-off of the pericardium. The works referred to at the end of this paper, 
and others, seem to show that the shutting-off of the anterior portion of the 
pericardial and pleural cavities from each other arises in a similar way 
in Mammals and Birds, in connexion with the ducts of Cuvier; while, on the 
other hand, such similarity cannot be claimed for the two classes in the matter 
of the closing-off of the posterior part of the pericardium from the rest of 
the body-cavity. In birds the closing-in of the pericardium posteriorly and 
postero-dorsally takes place, it appears, comparatively late; and finally in the 
adult we have the pericardium bulging into the peritoneal cavity, with little 
besides the peritoneum covering its postero-dorsal surface. In connexion with 
this subject, reference may be made to the recent paper by Strahl and Carius 
(9), where it is stated that in Mammals the part of the body-cavity which, 
later, forms the pericardial and pleural cavities arises distinct from the rest in 
the region of the “proamnion,” and that it becomes secondarily connected 
with the posterior part of the ccelome (a subsequent separation of course taking 
place). ‘These observers assert, as a further noteworthy distinction between that 
portion of the ccelome which in Mammals forms the pleural and pericardial 
cavities, and the corresponding part in Birds, the fact that in the former it is 
closed laterally, or on what is, when folding round of the sides takes place, 
the ventral side, whereas in the latter it is not so closed, but passes off into 
the extraembryoniccclome. Thus from the first the thoracic cavity would seem 
to be much better marked off in the case of Mammals than in that of Birds. 
2 The references in brackets are to the parts so indicated in the various 
figures on the Plates. 
