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Show 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 development1. 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 development of the " diaphragmatic " and " abdominal " air-sacs (s.a1, s.a", and s.a'"). (B) The development of the oblique abdominal septum (y), partly in connexion with the growth of the " abdominal air-sacs." (C) Tbe lateral extension of the ventral ligament of the stomach and hinder part of the liver (avian "omentum," (3), 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 ((3 + 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. («). 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, m a y be added with regard to the sbutting-offof 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 coelome (a subsequent separation of course taking place). These observers assert, as a further noteworthy 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 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 oft' into the extraembryonic coelome. Thus from the first the thoracic cavity would seem to be much better marked off in the case of M a m m a l s than in that of Birds. 2 The references in brackets are to the parts so indicated in the various figures on the Plates. |
