THE SKELETON 427 



then proceeds posteriorly, as the dorsal ends of the priraordia 

 rotate backwards and downwards towards the middle line. 



Although there are two lateral centers of chondrification, 

 these soon fuse. The carina arises as a median projection very 

 soon after concrescence in any region, and progresses backwards, 

 rapidly following the concrescence. There is, therefore, no stage 

 in which the entire sternum of the chick is ratite, though this 

 condition exists immediately after concrescence in any region. 

 The various outgrowths of the sternum (episternal process, antero- 

 lateral and abdominal processes), arise as processes of the mem- 

 branous sternum and do not appear to have independent centers 

 of chondrification. 



The sternum ossifies from five centers, viz., a median anterior 

 center and paired centers in the antero-lateral and abdominal 

 processes. The last appear about the seventeenth day of incu- 

 bation. On the nineteenth day a point of ossification appears 

 at the base of the anterior end of the keel. At hatching centers 

 also appear in the antero-lateral processes. The centers gradually 

 extend, but do not completely fuse together until about the third 

 month. The posterior end of the median division of the sternum 

 remains cartilaginous for a much longer period. In the duck 

 and many other birds there are only two lateral centers of ossifi- 

 cation; the existence of five centers in the chick is, therefore, 

 probably not a primitive condition. 



IV. Development of the Skull 



The skull arises in adaptation to the component organs of 

 the head, viz., the brain, the sense organs (nose, eye, and ear) 

 and cephalic visceral organs (oral cavity and pharynx); it thus 

 consists primarily of a case for the brain, capsules for the sense 

 organs, and skeletal bars developed in connection with the mar- 

 gins of the mouth and the visceral arches. In the chick, 

 the primordia of the auditory and olfactory capsules are con- 

 tinuous ab initio with the primordial cranium; the protecting coat 

 of the eye (sclera) never forms part of the skull. Therefore, we 

 may consider the development of the skull in two sections, first 

 the dorsal division associated with brain and sense organs (neuro- 

 cranium), and second, the visceral division or splanchnocranium. 

 Although the investment of the eyes forms no part of the skull, 

 yet the eyes exert an immense effect on the form of the skull. 



