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Show 706 MISS B. LINDSAY ON THE AVIAN STERNUM. [June 16, the sternum fully formed, lying in a position corresponding with that of the masses just described, and fused with the rib. The keel exists as a broad and thick longitudinal ridge, divided from the body of the sternum by a deep groove on the dorsal side. The coracoid has now acquired attachment to the sternum ; the clavicle is still shut off from the sternum by the pectoral muscle. The scapula does not join the coracoid to form a coraco-scapular plate, either at this or any other stage. A rudiment is apparent of the ligament joining the top of the coracoid with the top of the keel; and a very strong rudimentary ligament, reduced in later stages, unites the top of the scapula with the top of the keel. The anterior-lateral process is very thin and trilobed, which perhaps indicates an origin from three anterior ribs belonging to the three anterior muscle-bands indicated in the last stage. There are 7 sternal ribs. The outline of the fore limb at this stage indicates a division into three digits; it is shown in Plate XLIII. fig. 4 a, for comparison with the fore limb of the 7 days' Ostrich. 4th Stage, corresponding with a Chick of about 12 dags' incubation.- The lateral halves of the sternum are closed, the keel is fully formed, and the pectoral muscle has acquired the adult structure, namely, of three interconnected sheets. The clavicle is united to the keel, but dorsally a thin crack still indicates their junction (fig. 16, Plate XLV.). The anterior-lateral process of the last stage has been atrophied; this fact may be compared with that of the loss of the two anterior ribs at a subsequent stage. On the right side is seen the peculiar insertion of the coracoid characteristic of the adult. The posterior-lateral process of the metasternum has been added. No posterior reduction of the ribs is to be traced. In fact the posterior part of the sternum seems to retain what we may suppose to be a primitive type, as will be seen on comparison of the successive stages (figs. 15, 17, 18, Plate XLV.) with those of the Ostrich (figs. 3, 4, and 5, Plate XLIL). We may contrast these with the condition of the metasternum of the Chick, doubtless a form highly specialized in this respect as in many others ; in it, as we have seen, the processes of the metasternum are established from the first. 5th Stage, corresponding to a 14 days' Chick.-This shows the addition of the median process of the metasternum. The double apophysis of the median and anterior region is not added till a later stage. In the Chick, the median anterior apophysis is added between the end of the 9th and the beginning of the 14th day of incubation. Part III. CONCLUSIONS DRAWN FROM THE ABOVE DATA, in connection with facts previously known. 1. Conclusion drawn from the existence in the embryo of anterior ribs atrophied in the adult, of which some are seen attached to the sternum, and of muscle-bands apparently representing the intercostales |