470 MR. G. W. BUTLER ON THE SUBDIVISION OF [NoV. 19, 



general intestinal (or post-hepatic) cavity. In fig. 43 (which 

 represents the dorsal half of the trunk-region of one of these small 

 animals that had been cut horizontally), (z) indicates a blunt 

 wire or " seeker " passed through this channel of communication 

 from the post-hepatic cavity (3) to the pulmohepatic recess (2). 

 TJie posterior aperture {f-W) may be sometimes easily detected. 



It is the position of this channel that is specially noteworthy. 

 The opening into the post-hepatic cavity is not a proper foramen of 

 Winslow, as is the case in the bird up to the ninth day. It lies 

 dorso-externally to the dorsal attachment of the fatty so-called 

 " spleen," and has the same relation to the vena cava inferior, while 

 in the case of the foramen of Winslow the relations to the latter are 

 the reverse of this. On the other hand, the aperture here resembles, 

 in its relation to the vena cava inferior, the dextral foramen in the 

 post-hepatic septum of Tupinamhis {cf. p. 467, & figs. 31, 32, & 

 34, o), though in the case of this Lizard, owing to the absence of 

 pulmohepatic ligaments, there is no corresponding recess for it to 

 lead into (c/. woodcut B, p. 465). 



The omental sac proper (if, as is probable, one is originally 

 formed) would appear to have become obUterated in couuexiou with 

 the tight matting together of the coils of the alimentary canal in the 

 region of the stomacli and duodenum. 



I regret that I have failed to make out exactly what it is that 

 Huxley (4, p. 568) suggests may, in the Crocodile, represent the 

 oblique septum of birds. But it" the homologies advocated in this 

 paper be correct, the oblique septum, together with the pulmonary 

 aponeurosis, should be represented by the tissue covering the postero- 

 mesial face of the lungs veutrally, and on the riglit side forming the 

 antero-dorsal wall of the pulmohepatic recess. 



VI. Conclusions. 



1. The avian diaphragm is a single structure completed as such 

 (in the chick) about the tenth day of incubation and only subse- 

 quently divided into two parts by the growth of tlie " diaphragmatic " 

 or " intermediate" air-sacs. 



2. The facts described would appear to give support and defini- 

 tion to the view that the main part of the avian diaphragm is not 

 homologous with the diaphragm of mammals, but with tissue, which 

 in the latter group is called mediastinal. 



3. The post-hepatic septum of the bird is composed of two parts. 

 One, developing backwards and veutralwards from the posterior border 

 of the lung in connexion with the growth of the abdominal air-sacs, 

 seems to be quite rudimentary in other tj-pes which have not a like 

 disposition of air-sacs. The other, spreadmg on either side dorsally 

 and forwardly to meet the former, appears to be homologous with the 

 greater part or the whole of the post-hepatic system of the Crocodilia, 

 and of the Teiidce among Lizards. 



4. So far as the subdivision of the body-cavity is concerned, the 

 Crocodiles seem comparable to the Lizards rather than to the Birds. 



