OCR Text |
Show 698 MISS B. LINDSAY ON THE AVIAN STERNUM. [J the edges of both processes. The second process is already indicated in the earliest embryos examined, so that it is clearly formed by addition, not by absorption. Plate XLIII. fig. 19, by the presence of the additional filling absent in the earlier embryos, shows an indication that the increased size of the pectoral muscles in birds that fly well demands increased breadth of their sternal attachments, and thus leads to the filling up of the sternum, so that the processes are obscured by their own growth, and finally become confluent. 5. In some, but not all, of the older specimens there was found a 7th sternal rib. 6. The condition of the anterior-lateral part of the sternum, which gives attachment to ribs anteriorly to the base of the coracoid, is to be contrasted on the one hand with the anterior-lateral process of the Ostrich, formed as a secondary addition to the rounded outline of the costal sternum, and on the other hand with that of the Chick, in which an apparent process of the costal sternum is first formed through the loss of two anterior ribs primitively attached and afterwards augmented by a secondary growth. An old five days' embryo, the earliest examined, calls for a special description. The coracoid (cf. Plate XLIII. fig. 8) exhibits a remarkable resemblance to that of the Ostrich ; the median depression, however, does not amount to a foramen, and it is filled by a separate mass of tissue, attached to the coracoid itself by embryonic muscular tissue. This specimen, which is quite normal and alike on both sides, seems to compel the conclusion that the coracoid of the adult Gull represents a fusion of the true coracoid with the precoracoid, in which the foramen which remains open in the case of the Ostrich is filled up by further ossification-a conclusion sufficiently startling, notwithstanding that so great an authority as Parker has already combated the assumption that the elements of the shoulder-girdle are the same in all birds. The scapula, as will be observed in the Chick at a similar age, is quite free from the coracoid. This embryo presents a median mass of tissue, corresponding with the position of the top of the keel. The interpretation of this as an interclavicle is forbidden by the position of the clavicles, which are, as seen in the diagram, very small, far from the median line, and not even directed towards it; further, this centre, separated from the pericardial cavity only by the thinnest connective tissue, occupies a less superficial position than the clavicles, which overlie a thick stratum of embryonic muscle. It will be seen, on the other hand, that this centre arises in closest connection with the already approximated sternal halves. Comparison with a certain occasional median cartilage in the Chick will show that too much importance must not be attributed to this centre in the Gull; the former is also median, but its posterior position shows that it cannot be an interclavicle. It must therefore be a formation of recent date, and its occasional appearance shows that the keel has a tendency, not yet established, to differentiate itself from the rest of the sternum : this tendency is expressed, in a much lower degree, by the existence of a separate centre of ossification for the keel in the Gull and Chick, as in manv |