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Show 700 MISS B.LINDSAY ON THE AVIAN STERNUM. [June 16, intercostales externi and (b) a primitive arrangement are the following:- (a) (I). Their correspondence in number with the ribs and inter-costal position in the case of the Gull and Chick (2). Their subsequent fusion into a supercostal sheet, au-ferentiated finally into the intercostales externi. (b) (1). Their curvature, slight as compared with that ot the ribs. (2). Their extent beyond corresponding ribs, in the case ot the cervical ribs of the Chick, and posterior ribs ot the Chick and Ostrich. It is to be hoped that further research may elucidate the origin and exact relations of these bands ; the inquiry must be attended with some difficulty, however, since the mesoblast has but recently acquired histological differentiation when they present the appearances described. e . 4. The evidences of the shortening of the sternum are the following :-the adult condition presenting five ribs with uncinate processes and two long unattached ribs without them, one anterior and one posterior. (a) Anterior shortening. (i.) Chicks of 7 days showed in a number of cases the elongation of 1-3 ribs anterior to the first of those named above ; none of these reach the sternum. (ii.) Chicks early on the 6th day invariably showed two at least of these ribs fully attached to the sternum ; they are very small and close together. (iii.) The anterior muscle-bands, previously described, which atrophy from their spinal ends onwards, suggest that their corresponding ribs have passed into the sternum and disappeared by a process of atrophy like that already noticed in the Ostrich. This view is supported by the condition of a 5 days' Chick, which shows four of the muscular rudiments named overlying four minute masses of cartilage near the median line ; this specimen is in many respects abnormal, but certainly these rudiments suggest the former existence of a pre-sternum. (b) Posterior shortening. In about one out of every four Chicks from 6 to 8 days incubated, there is a rudiment of a posterior 8th rib, atrophied at both ends as in the Guillemot; in earlier stages this rib is seen, but not so frequently, attached to the spinal end, or even at both ends. Before this rudiment disappears it approaches very close to the next rib, apparently because the intercostal muscle uniting them does not grow so fast as those elsewhere, and in one case the rudiment is seen attached to the next rib, forming a process (cf. Fig. III. diagr. a, p. 708). The 7th rib itself is often attached to the sternum up to the end of the 9th day. The condition of the ribs in the ancestor of the Fowl, thus shown, may be compared with that of allied forms, the Waterhen and the |