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 alter- 

 nate 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 hare any value assigned to them, it must be remembered that 

 early on the 5th day ar/gregation 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 to- 

 wards 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 G days' Chick with a sternum quite 

 abnormal, in which two other processes appear. Possibly they may 

 be formed as rudiments of the 7th and Mth 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 baiids, of which four lie ventral to the metasteruum 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, ihe 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 Gth 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 fidl 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 



