1889.] THE BODY-CAVITY IN LIZARDS, ETC. 4.^)5 



referred to, between which lie the two pairs of air-sacs that he calls 

 reservoh-s diaphragmatiques or moyens {intermediate air-sacs of 

 Huxley), as constituting a single diaphragm ; and Milne-Edwards 

 says {loc. cit.), " I'appareil diaphragmatique des oiseaux se compose 

 de deux portions ou diaphragmites." This unity of the wliole 

 apparatus, which is very apparent when the development is followed 

 {cf. § III.), cannot, I think, be too clearly kept in view, especially 

 when homologies are being discussed. 



In this paper I refer to all from the anterior or dorsal face of the 

 pulmonary aponeurosis to the peritoneum covering the posterior or 

 ventral face of the oblique septum, as the Avian diaphragm, in con- 

 tradistinction to the Alammalian diaphragm, with which, in agree- 

 ment with Huxley's verdict, it is argued in the sequel it has little, 

 if any, true homology. 



With regard to the term " oblique septum " of Huxley. As he 

 speaks (p. 562) of the four post-bronchial air-sacs of either side as 

 being shut off by the " oblique septum " in a similar kind of way, I 

 presume that he includes under this term the septum (y in the 

 Plates) dorsal to the liver, above referred to. At all events in using 

 this term, I refer merely to that septum which in the Fowl is (cf. 

 p. 458), so to speak, blown away from the other part of the Avian dia- 

 phragm by the growth of the intermediate, or diaphragmatic, air-sacs. 



I would call this septum (y), which on either side forms the 

 dorsal wall of the pulmohepatic recess, and into which a large part 

 of the abdominal air-sac projects', the oblique abdominal septum. 

 It is true that the more lateral parts of this septum, in that they 

 take their final form in connexion with the abdominal air-sac, might 

 so far be held to resemble the " oblique septum " proper, in its 

 relation to the diaphragmatic air-sac ; but this would not apply to the 

 more median parts of the septum (y), and the nature of the two things 

 is really very different. That which I have termed the oblique abdo- 

 minal septum does really separate one part of the body-cavity from 

 another, as described bySappey(footnote, p. 454 above), and is covered 

 on both sides by peritoneum ; but the " oblique septum " is only part 

 of a coelomic septum, the other part being the pulmonary apo- 

 neurosis. 



The oblique abdominal septum we might perhaps regard as a 

 backward continuation of the whole avian diaphragm with the abdo- 

 minal air-sacs between its dorsal and ventral laminae, just as the 

 intermediate or diaphragmatic air-sacs lie between the two laminae of 

 the latter. 



III. On the Development and Homologies of the 

 VARIOUS Septa in the Body-cavity of the Chick. 



We may now turn to consider the development of the septa in 

 the body-cavity of the Fowl, to which reference has been made in 

 the preceding section. 



1 Sappey (see footnote, p. 454) says that these air-sacs red on (s'appuienfc 

 sur) the septum in question ; but it is more correct to say that they project into 

 it, in the end, as it were, blowing away a dorsal lamina from the rest. 



