456 THE DEVELOPMENT OF THE CHICK 
procoracoid component does not develop beyond the membranous stage 
in the chick. It is interesting that the clavicle is the first center of ossi- 
fication in the body, though perichondral ossification of some of the 
long bones begins almost as soon. 
The Wing-bones. The primordium of the wing-bones_ is 
found in the axial mesenchyme of the wing-bud, which is origi- 
nally continuous with the primordium of the pectoral girdle, and 
shows no trace of the future elements of the skeleton. The 
differentiation of the elements accompanies in general the external 
differentiation of the wing illustrated in Figs. 121 to 124, Chapter 
VII. The humerus, radius, and ulna arise by membranous differ- 
entiation in the mesenchyme in substantially their definitive 
relations; they pass through a complete cartilaginous stage and 
Fig. 246. — Photograph of the pectoral 
girdle of a chick embryo of 274 hours; 
prepared by the potash method.  (Prep- 
aration and photograph by Roy L. 
Moodie. ) 
1, Coracoid. 2, Clavicle. 3, Scapula. 
4, Humerus. 
then ossify in a perichondral fashion (see Fig. 242). In the 
carpus, metacarpus, and phalanges, more elements are formed 
in the membrane and cartilage than persist in the adult. Elimi- 
nation as well as fusion takes place. These parts will therefore 
require separate description. 
As birds have descended from pentadactyl ancestors with 
subsequent reduction of carpus, metacarpus, and phalanges, it 
is naturally of considerable interest to learn how much of the 
ancestral history is preserved in the embryology. The hand is 
represented in the embryo of six days by the spatulate extremity 
of the fore-limb, which includes the elements of carpus, meta- 
carpus, and phalanges. From this expansion five digital rays 
grow out simultaneously, the first and fifth being relatively 
