OCR Text |
Show 1885.] MISS B. LINDSAY O N T H E AVIAN STERNUM. 703 It is always stated that the coracoid and scapula of the Chick arise as a single plate, but the various specimens examined did not bear out this description. The shoulder-girdle of the 5 days' chick, in which the pericardial cavity is not quite closed, is shown in figs. 4 and 5, Plate X L I V . It consists of three pieces, whereof the middle one is partly attached to that nearest the median line. According to Gotte, w h o maintains the usual view that the scapula and coracoid form a single plate, and who further describes the episternal projection of a 5 days' Chick as clearly distinguishable from the clavicle itself, these three pieces would be interpreted as coraco-scapular plate, clavicle, and interclavicle; but this interpretation, rendered doubtful by the early separation of scapula and coracoid, already noticed in the Ostrich and the Gull, is rendered impossible by comparison of subsequent stages. Plate X L I V . figs. 3, 4, 5, and 6, show how the median piece gradually elongates, without growing thicker, and forms the clavicle, while the intermediate and the dorsal piece fuse to form the coraco-scapular mass usually described, which was found to exist early on the 6th day (fig. 6 ) . The late date of its appearance suggests that this fusion m a y represent rather a Struthionic than a pre-Avian condition. The interpretation as clavicle of the median piece which is partially attached to the intermediate one, agrees with the description of Rathke, who says that at an early stage the coracoid and clavicle are united dorsally but not ventrally; nor is it inconsistent with that of Gotte himself, w h o says the end of the coracoid passes under the clavicle, which is the same thing in other words. Now the median piece, though posteriorly free, is united anteriorly to the coracoid; and this anterior union is dorsal, for the two pieces appear separated by a crack when seen from the surface (cf. figs. 3 and 4), yet on dissection they are found united below. At first sight the relative positions of the two pieces present a difficulty in the interpretation just given, viz., that they are placed end to end, not parallel as in the adult, so that the clavicle is separated from the scapula by the whole length of the coracoid. The same difficulty, however, attends Gotte's account of the relation of the two parts; since, although he traced the end of the coracoid under the outer end of the clavicle, he expressly states that he failed to trace it far towards the median part of the clavicle ; in other words, he also found the two placed, at this stage, end to end. Again, a second difficulty lies in the fact that the intermediate piece of the shoulder-girdle is triangular, which the coracoid is not. During the 5th day, however, it loses this triangular shape, and is no longer found to be, as at first, a flat mass, comparatively thin. This change suggests that possibly the anterior apex of the triangle is equivalent to the precoracoid of the 5 days' Gull (which also forms a mass lying between the clavicle and scapula), and that the clavicle is gradually carried up towards the dorsal end, by the atrophy of that part, with which it is in close connection. This theory is best explained by the following series of diagrams. Reference to the Plates will show that, although themselves diagrammatic, these figures |