THE SKELETON 



437 



small; the second, third, and fourth represent the persistent digits. 

 In each ray is a membranous skeletal element, which, however, 

 soon disappears in the first and fifth. Thus there are distinct 

 indications of a pentadactyl stage in the development of the 

 bird's wing.. 



In the definitive skeleton there are but two carpal bones, 

 viz., a radiale at the extremity of the radius, and an ulnare at 

 the extremity of the ulna. In the embryo there is evidence of 

 seven transitory pieces in the carpus arranged in two rows, proxi- 

 mal and distal (Fig. 247). In the proximal row only two car- 



Mc.J 



M'c.2 



P'cfi. 



17. 



FIG. 247. Skeleton of the wing of a chick embryo of 8 days. (After W. 

 K. Parker.) 



Cp. 2, 3, and 4, Second, third, and fourth carpalia. C. U., Centralo- 

 ulnare. H., Humerus. I. R., Intermedio-radiale. M'c. 2, 3, 4, Second, 

 third, and fourth metacarpalia. P'ch., Perichondral bone R., Radius. 

 U., Ulna. 



tilages appear, viz., the radiale and ulnare; but in earlier stages 

 each appears to be derived from two centers: the radiale from a 

 radiale s.s. and an intermedium, the ulnare from an ulnare s.s. 

 and a centrale. Evidence of such double origin of each is found 

 also in the cartilaginous condition (v. Parker, 1888). Four 

 elements in all enter into the composition of this proximal row. 

 In the distal row there are three distinct elements corresponding 

 to the three persistent digits, and representing, therefore, carpalia 

 II, III, and IV. These subsequently fuse with one another, 

 and with the heads of the metacarpals to produce the carpo- 

 metacarpus. 



On the seventh day the metacarpus is represented by three 

 cartilages corresponding to the three persistent digits, viz., II, 



