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 tlie 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 dev'elopment of the oblique abdominal septum (y), 

 partly in connexion with tlie growth of the " abdominal 

 air-sacs." 



(C) The lateral extension of the ventral ligament of the stomach 

 and hinder part of the liver (avian " omentum," (i), so 

 that meeting the pulmohepatic ligametits (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 (jS -{- 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 ligatnents (a, a). 



III. (a). On the Development of the Air-sacs in relation to the 

 parts of the Avia?i Diaphragm. 



On the 8th day, while the passages between the pleural and 

 peritoneal cavities are widely open, the abdominal air-sac first 



' The following remarks, however, may be added with regard to the 

 shuttii)g-oiFof 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 chxsses 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 

 tliis 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). Tliese observers assert, as a further notewortliy distinction between that 

 portion of the coelome which in Mammals forms the pleural and pericardial 

 cavities, and the corresponding part in Birds, the fact that iu 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 extraembryonic ccelome. Thus from the first the thoracic cavity would seem 

 to be much better marked ofi' in the case of Mammals than in that of Birds. 



- The references in brackets are to the parts so indicated in the vai-ious 

 figures on the Plates. 



