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- 
Fic. 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. 
UU; 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 eartilaginous 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, Ill, 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, 
