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Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 701 Rail. The posterior shortening is evidently of much more recent date than that just described at the anterior end. 5„ There appeared indications of the compound character of the sternum. Several 5 days' Chicks, in which the pericardial cavity was not quite closed, exhibited, as already mentioned, thickenings alternate with the sternal ends of the ribs. Sowewhat later 5 days' Chicks showed (in that anterior part which by the end of the 5th day gives attachment to the coracoid) irregular thicker masses incompletely united. If these traces of structure in the sternum appear somewhat slight to have any value assigned to them, it must be remembered that early on the 5th day aggregation of tissue is almost the only sign of differentiation of the mesoblast. Such aggregation is frequently seen better in dissections than in sections. 6. The posterior processes of the sternum are mapped out very early. In the Chick a newer process is added on the lateral aspect of the posterior-lateral process, whereas in the Gull one is added towards its median aspect. This process, the external xiphoid, appears in a 5 days' Chick as a minute outgrowth from the posterior-lateral process; its development may be traced by comparison of the diagrams. Plate XLV. fig. 10 shows a 6 days' Chick with a sternum quite abnormal, in which two other processes appear. Possibly they may be formed as rudiments of the 7th and 8th ribs previously attached to the sternum ; but the whole structure of this specimen is abnormal, especially in the following respects-(1) The position of the primitive intercostal bands, of which four lie ventral to the metasternum and nearly meet over it in the median line ; (2) the condition of its coracoid aspect; and (3) the four rudiments in the neck, already described. Finally, there remain to be discussed, 7, the formation of the keel, and 8, the evidences for or against the existence of an interclavicle. 7. In three Chicks out of the thirty younger specimens, there was found a separate cartilaginous centre for the keel. The ages of these three were 7 days. Two of the three centres are figured, to show that their posterior position precludes their being interpreted as interclavicles; the third was precisely similar; the clavicles at this stage are still open. With these exceptions the keel was found to appear with the fusion of the halves of the sternum ; these approach late on the 6th day and fuse gradually from the 9th till the beginning of the 10th day ; as the fusion passes backwards, so does the keel. It is at first low, but afterwards acquires its full height. These facts, taken in connection with that long ago established by L'Herminier, that ossification of the keel commences at its base (where the keel is not, as in the Duck and Heron he states it to be, ossified by fusion of paired centres in the sternum), point to the conclusion that the keel is a structure formed by the fused edges of the halves of the sternum. Its centre of ossification is, according to the same authority, the last to appear in the sternum of the Chick. Gotte's account of the |