698 MISS B. LINDSAY ON THE AVIAN STERNUM. [JunC 16, 



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 indi- 

 cation 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, hut 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 isfirst 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 depres- 

 sion, 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 precora- 

 coid, 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 tlie 

 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 vnth 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 many 



