IS^O.] MtSS B. LINDSAY ON THE AVIAN STERNUM. 709 



this one case shows the sternum on the way to become filled 

 up. For other cases of apparent filling-up see diagrams 1, 2, and 3, 

 fig. III. 



4. Conclusion from the late date of its appearance and from 

 the usual absence of any anterior median rudiment, pre-existing 

 before the closure of the sternal halves : — that the keel is an out- 

 growth of the sternum of comparatively late phylogenetic date, and 

 created for and by the attachment of the pectoral muscles. 



5. Conclusion drawn from their late appearance in the enbryo, 

 later than the commencement of the keel : — that the median furcular 

 apophysis, and the occasional median anterior sternal apophysis, are 

 similarly structures of a late date, and entirely without connection 

 with any ancestral interclavicle. 



6. Conclusion from the comparison of embryo, of 4 days' incuba- 

 tion in the Ostrich, and 5 in the Gull and Chick, and from the com- 

 parison of the precoracoid in adult forms : — that the complex relations 

 of the parts of the shoulder-girdle are not to be interpreted as 

 indicating the existence of an interclavicle, but are rather due, in 

 some types at least, to the existence of a rudimentary precoracoid. 



7. Conclusion drawn from comparison of Chick and Gull : — that 

 the anterior lateral process of the sternum is not always of the same 

 nature, being certainly an outgrowth of the costal sternum in the 

 Ratitse, while in other types it is apparently the rudiment of a former 

 anterior extent of the costal sternum. 



The progress of ossification in the sternum, as observed by 

 L'Herminier, bears out the above conclusions. The " mesosternals " 

 are the part earliest ossified, these centres occupying the lateral 

 region ; while the middle part (Parker's Lophosteon), which we have 

 seen added to the costal sternum, as the median part of the 

 niesosternum, in t!ie case of the Ostrich, ossifies at a later date. It 

 is to be remarked that in the Ostrich one pair of ossifications alone 

 exists, the " mesosternals," above interpreted as the primitive costal 

 part of the sternum ; these centres are quite lateral in position. lu 

 the Goose, the lateral position of the ossific centres is still more 

 remarkable ; they are in fact quite marginal, occupying the region 

 which Huxley calls the " costal process." These facts confirm the 

 above theory, that the median part of the sternum is of later forma- 

 tion than the primitive costal bands. 



In connection with the subject of ossification, it is necessary to 

 deprecate the undue importance sometimes attributed to the 

 existence of a special centre of ossification for the keel. Little 

 phylogenetic value can be attributed to the existence of separate 

 centres, except in the case of a bone which is in course of retrogres- 

 sion ; in a bone like the Avian sternum, of still increasing importance, 

 ncAV centres tend to appear, representing not phylogenetic facts, but 

 the positions of greatest strain and greatest strength in the bone at 

 present. Among many familiar instances of the distinction that 

 must be made in the two cases, are, on the one hand, the separate 

 ossicles of the hyoid apparatus in the higher Mammalia, or the 

 nucleus of the coracoid process in Man, and, on the other hand, the 



Proc. Zool. Soc— 188.1, No. XLVI. 46 



