OCR Text |
Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 709 this one case shows the sternum on the way to become filled up. For other cases of apparent filling-up see diagrams 1, 2, and 3, fig. III. 4. Conclusion from the late date of its appearance and from the usual absence of any anterior median rudiment, pre-existing before the closure of the sternal halves:-that the keel is an outgrowth of the sternum of comparatively late phylogenetic date, and created for and by the attachment of the pectoral muscles. 5. Conclusion drawn from their late appearance in the embryo, later than the commencement of the keel:-that the median furcular apophysis, and the occasional median auterior sternal apophysis, are similarly structures of a late date, and entirely without connection with any ancestral interclavicle. 6. Conclusion from the comparison of embryo, of 4 days' incubation in the Ostrich, and 5 in the Gull and Chick, and from the comparison of the precoracoid in adult forms :-that the complex relations of the parts of the shoulder-girdle are not to be interpreted as indicating the existence of an interclavicle, but are rather due, in some types at least, to the existence of a rudimentary precoracoid. 7. Conclusion drawn from comparison of Chick and Gull:-that the anterior lateral process of the sternum is not always of the same nature, being certainly an outgrowth of the costal sternum in the Ratitae, while in other types it is apparently the rudiment of a former anterior extent of the costal sternum. The progress of ossification in the sternum, as observed by L'Herminier, bears out the above conclusions. The " mesosternals " are the part earliest ossified, these centres occupying the lateral region; while the middle part (Parker's Lophosteon), which we have seen added to the costal sternum, as the median part of the mesosternum, in the case of the Ostrich, ossifies at a later date. It is to be remarked that in the Ostrich one pair of ossifications alone exists, the " mesosternals," above interpreted as the primitive costal part of the sternum ; these centres are quite lateral in position. In the Goose, the lateral position of the ossific centres is still more remarkable ; they are in fact quite marginal, occupying the region which Huxley calls the " costal process." These facts confirm the above theory, that the median part of the sternum is of later formation than the primitive costal bands. In connection with the subject of ossification, it is necessary to deprecate the undue importance sometimes attributed to the existence of a special centre of ossification for the keel. Little phylogenetic value can be attributed to the existence of separate centres, except in the case of a bone which is in course of retrogression ; in a bone like the Avian sternum, of still increasing importance, new centres tend to appear, representing not phylogenetic facts, but the positions of greatest strain and greatest strength in the bone at present. Among many familiar instances of the distinction that must be made in the two cases, are, on the one hand, the separate ossicles of the hyoid apparatus in the higher Mammalia, or the nucleus of the coracoid process in Man, and, on the other hand, the P R O C ZOOL. Soc-1885, No. XLVI. 46 |