126 STRUCTURE AND CLASSIFICATION OF BIRDS 



ment, of which one (apparently the middle one) is not 

 recognisable in the adult. 



LEIGHTOX has contributed to this question with a study 

 of the development of the wing of Sterna. 1 He finds here 

 too a rudimentary fourth digit, which in the first stage, 

 which he figures, is as long or nearly as long as the first 

 digit. A rudimentary metacarpal even persists attached to 

 the side of the last metacarpal in birds just before hatching. 

 In the carpus there are never more than four distinct 

 cartilages ; there are in the first place a radiale and ulnare, 



Fio. 71. DK;ITS OF OSTRICH (AFTEK WRAY). 



1, phalanx 1 (/'/<!) arid rudimentary phalanx 2 (7V<2) of digit III. ; c, connective tissue. 

 2, another specimen with phalanges ankylosed. 3, distal part of digit III. of 

 embryonic manus (4). 



and distally two cartilages, of which one appears to represent 

 the combined distalia of the two first digits, and the second 

 that of the third digit. In birds, just before hatching, all 

 the distalia have fused into one mass. The cartilages 

 lettered respectively radiale and ulnare in the figures are 

 thought, however, by the author to be really radiale + 

 intermedium and ulnare + centrale ; and in support of this 

 view is the partial separation between the two supposed 

 elements of each, which is, however, never carried very 

 far. 



As to the homologies of the digits in the adult with those 



1 ' The Development of the Wing of Sterna Wilsonii,' Tufts Coll. Studies, 

 1894. Previous literature is here quoted. 



