;jl4. TROPICAL WILD LIFE IN BRITISH GUIANA 



proximal portion of the third digit which iiiay be the rudi- 

 nieiit of a fourth. In exaniiniim" tliis bone in an embryonic 

 king vulture, I found, as in the ani, that there is a partially 

 free jirotuberance near its proximal end. Unfortunately, 

 however, having no younger embryo, I cannot say if the 

 ])n)tubcrance is ever entirely separate in tliis l)ird. 



All things considered it would seem, in the ani, as if this 

 ])rojection is the remnant of a foin'th digit or one of its car- 

 ])als that at one time was separate, but now has become at- 

 tached to the third. That it is not a later specialization of the 

 third is shown by the fact that only at a fairly early embry- 

 onic stage is it at all free. If it were a specialization, it 

 never would have been entirely so. 



On the other hand, figuring tliat the stem of the T is 

 the remnant of a fourth digit it woidd seem as if there might 

 be some significance attaclied to the flattening and near di- 

 vision of the third metacarpal. This does not seem to be 

 true, however, as up to the time the chick is several weeks 

 old, the bone remains round and does not really commence 

 to flatten until the bird is able to fly. It is a recent specializa- 

 tion caused, possibly, by some individual movement of the 

 wing and does not have any direct bearing on the irregular 

 structure of the third digit. 



GROWTH or THE AVING 



A significant fact about the wing is the rate of growth 

 of the various elements of the hand. The hand is divided 

 into two component parts, metacarpus and digitus. Com- 

 pared with the human hand, the metacarpus represents the 

 palm, and the digitus, the thumb, first and second fingers, 

 or as some will have it — the first, second and third fingers. 

 Let lis call the parts of the wing in question the palm and 

 the index. 



Figuie lOOa is a curve drawn to represent the growth of 

 these parts starting w^ith the half grown embryo. Tlie length 



