THE PECTORAL GIRDLE AND FORE-LIMB. 333 



manus show peculiar modifications in all birds, and are of much 

 greater interest (Fig. 130). 



a. The carpus. On the seventh day the carpus consists of: 

 (i) a proximal row of two cartilages, of which the larger one is 

 situated opposite the end of the radius, and is regarded by 

 Parker as corresponding to the radiale and intermedium of the 

 typical carpus ; while the smaller one, placed opposite the end 

 of the ulna, is commonly regarded as the ulnare, but, according 

 to Parker, corresponds to the ulnare and centrale of the typical 

 carpus, (ii) A distal row of two cartilages, a larger one on the 

 radial side and a smaller one on the ulnar side. A little later, 

 about the tenth day, the radial cartilage divides into two, giving 

 three cartilages in the distal row of the carpus, of which the 

 middle one is the largest. 



The carpus remains in this condition until some time after 

 the hatching of the chick. The two proximal carpals persist as 

 the two free carpals of the adult bird ; they begin to ossify in 

 chicks about five weeks after hatching. The distal carpals 

 remain free for some time, but ultimately, in chicks eight or 

 nine months old, they unite with the metacarpals. 



b. The digits. In the manus of the fowl there are three well- 

 formed digits, which at first are quite independent of one 

 another, and which correspond to the three radial digits, pollex, 

 index, and medius, of the typical Vertebrate manus. The fourth 

 digit is rather doubtfully represented by a small rudiment. 



On the seventh day the first three metacarpals are well- 

 developed cartilaginous rods, completely separate from one an- 

 other, and from the carpus. The first metacarpal is short ; the 

 second long and thick ; the third about the same length as the 

 second, but much thinner. The first digit or pollex has two 

 short phalanges ; the second digit or index has three ; and the 

 third digit or medius has two, of which the terminal one ulti- 

 mately disappears. 



During the tenth day a small nodule of cartilage, the pre- 

 pollex, appears on the radial side of the first metacarpal, with 

 which it ultimately fuses. Another small bar of cartilage ap- 

 pears, about the same time, on the outer side of the third meta- 

 carpal, at its proximal end ; this, which perhaps represents the 

 fourth metacarpal, remains distinct until some time after hatch- 

 ing, ultimately fusing with the base of the third metacarpal. 



