LIMBS 



167 



In the carpus, at least seven elements are recognisable. In 

 the proximal row is an intermedio-radiale and a centro-ulnare, 

 each of which consists of two parts in the embryo. In the distal 

 row there are also two elements, one of which (carpale 2 + 3) is 

 evidently primarily double : the other corresponds to carpale 4. 

 In early stages four distinct metacarpals can be seen, and these 



-Ktauc 



FIG. 133. CARPUS OF EMBRYO OF Sterna ivilsoni. A, STAGE AT WHICH 

 OSSIFICATION BEGINS ; B, STAGE IMMEDIATELY HEFORE HATCHING. (After 

 V. L. Leighton.) 



c, distal carpals ; Klaue, claw ; Rod, radius ; rad, interinedio-radiale : Uln, 

 ulna; uln, centro-ulnare; // V, metacarpals (in B, /Fand V have become 

 fused). 



seem to correspond to the 2nd-5th rather than to the lst-4th : the 

 5th metacarpal soon fuses with the 4th (Fig. 133). 



The distal carpals become fused with the corresponding meta- 

 carpals, thus forming a carpometa carpus (Figs. 132, 133), and in 

 the adult only the two proximal elements remain separate as a 

 radiale and an ulnare. The three metacarpals themselves become 

 united proximally, and the second (///) and third (IV) distally: 

 they only bear a limited number of phalanges at their free ends. 



Claws were present on the terminal phalanges of all three 

 digits in Archseopteryx (Fig. 49). In certain recent adult Birds 

 (e.g. Chauna) the first digit (//) bears a claw, and more rarely 



