THE SKELETON 435 



out in front of the coracoid process ventrally and towards the 

 middle Une. These processes are quite well developed on the 

 fifth day, and increase considerably in length on the sixth day, 

 when the hind end of the scapula nearly reaches the anterior end 

 of the ilium, and the lower end of the coracoid is very close to 

 the sternum. The elements are still continuous in the glenoid 

 region. 



About the end of the sixth day independent centers of chon- 

 drification appear in the scapula and coracoid respectively near 

 their imion; these spread distally and fuse centrally, so that 

 on the seventh day the coraco-scapula is a single bent cartilagi- 

 nous element. In the angle of the bend, however (the future 

 coraco-scapular joint), the cartilage is in a less advanced condi- 

 tion than in the bodies of the two elements. The clavicular 

 process, on the other hand, never shows any trace of cartilage 

 formation, either in early or more advanced stages, but ossifies 

 directly from the membrane. It separates from the other ele- 

 ments of the pectoral girdle, though not completely, on the eighth 

 day. 



The scapula and coracoid ossify in a perichondral fashion, 

 beginning on the twelfth day, from independent centers, which 

 approach but never fuse, leaving a permanent cartilaginous 

 connection (Fig. 242). The clavicle, on the other hand, is a 

 purely membrane bone; bony deposit begins in the axis of the 

 membranous rods on the eighth or ninth days, soon forming 

 fretted rods that approach in the mid-ventral line by enlarged 

 ends, which fuse directly without the intervention of any median 

 element about the twelfth to thirteenth day, thus forming the 

 furcula or wish-bone (Fig. 246). 



The nature of the clavicle in birds has been the subject of a sharp 

 difference of opinion. On the one hand, it has been maintained that it 

 is double in its origin, consisting of a cartilaginous axis (procoracoid) 

 on which a true membrane bone is secondarily grafted (Gegenbaur, Fiir- 

 bringer, Parker, and others) ; on the other hand, all cartilaginous preforma- 

 tion in its origin has been denied by Rathke, Goette, and Kulczycki. After 

 careful examination of series of sections in all critical stages, and of 

 preparations made by the potash method, I feel certain that in the chick 

 at least there is no cartilaginous preformation. It is still possible (in 

 deed probable on the basis of comparative anatomy) that the theory 

 of its double origin is correct phylogenetically; but it is certain that the 



