THE SKELETON 427 
then proceeds posteriorly, as the dorsal entls of the primordia 
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. 
