436 



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 



