THE SKELETON 435 
out in front of the coracoid process ventrally and towards the 
middle line. 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 ccracoid 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 union; 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, Fur- 
bringer, Parker, and others) ; on the other hand, all cartilaginous preforma- 
tion in its origin has been denied by Rathke, Goette, and Kulezycki. 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 
