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Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 705 former view rather than the latter. Even in Casuarius it is only occasional (cf. fig. 8). The same uncertainty must exist with regard to the precoracoid of Diomedea (fig. 1, Plate XLIV.) ; although there can be little doubt, from comparison with the early stages of the various embryos figured, that we must agree with Sabatier in regarding this region as the Avian precoracoid, rather than the precoracoid of Parker, which Sabatier calls an epiphysis (cor. ep., fig. 1). Under these circumstances it would be hazardous, notwithstanding the Struthionic aspect of the coracoid in the Gull, to put forth with any certainty a view that the pre-Avian kind of precoracoid is to be identified with the region suggested as precoracoid in the Chick ; but at the same time the development of this marginal region in the types figured renders it extremely likely that the region already described in the shoulder-girdle of the late five days' Chick is equivalent to a precoracoid of some kind, though possibly of a secondary character, developed in the more immediate ancestors of the Gallinacese. (v.) T H E GANNET. (9 specimens, from the bird just hatched to a stage comparable with a 5 days' Chick.) The sternum of the adult Gannet, it may be observed, since it possesses a furcula continuous with the keel, and behind it a median paired apophysis for the support of ligaments, exhibits at once the interclavicle of Gotte in front, and the interclavicle of Harting at a more dorsal level. In this bird, if in any, we might expect to trace a genetic connection between clavicle and keel, since in the adult these parts are fused. Such, however, is not the case. The embryo is particularly easy to examine, because it is so large that the earliest aggregations in the mesoblast can be clearly traced. The following were the stages observed, recorded, for the sake of convenience, in inverse order, youngest first. 1st Stage, corresponding with a five days' Chick.-There are 8 ribs, which end freely, and no trace of sternum. The three parts of the shoulder-girdle are separate and almost parallel, overlying a remarkably thick mass of tissue representing the pectoral muscle. 2nd Stage, corresponding with a later five days' Chick.-The sternum is indicated by thick opaque aggregations in a layer of mesoblast, shown in fig. 14, Plate X L V. A large mass indicates the sternum, a smaller one the keel, while three smaller masses, which in the next stage have disappeared, may probably be compared with the three anterior muscle-bands previously described in the Chick. The end of the clavicle, which is of course superficial, lying next to the skin, is completely shut off from the deeper layer, in which lie the differentiating sternum and keel, by the whole thickness of the pectoral muscle, a depth nearly half the length of the clavicle itself. 3rd Stage, corresponding with a six days' Chick.-This shows |