160 Degeneration in the Ostrich 



conspicuously the second phalanx is however a distinct bone, nearly 

 an inch in length, articulating with the first, its free end knob-like in 

 correlation with the callosity which it bears. But all intermediate sizes 

 can be procured, corresponding with the varying sizes of the digit as 

 seen from the outside. 



Among the large number of ostriches available for study, we 

 therefore find numerous variations which serve to illustrate how the 

 degeneration of the third digit is proceeding, and these are supple- 

 mented by embryonic stages. In the adult no external trace of the 

 digit may be left, while again it may project as a distinct finger ; also 

 the second phalanx may be represented by intervening stages from a 

 mere nodule to a bone an inch or so in length. Different individuals 

 have reached different stages in retrogression, but the passage from 

 the one to the other is by barely perceptible stages, not by con- 

 spicuous steps. 



In the embryo ostrich, as Dr R. Broom ^ has first shown, traces of 

 the first three digits of the wing occur at about eleven days' incubation 

 and also a vestige of the fourth metacarpal. The latter however persists 

 for a very short time, and no trace of it appears in the late or newly 

 hatched chick. The factors concerned in the partial production of the 

 fourth manifestly still occur in the germ, but in a very reduced and 

 weakened condition compared with the ancestral reptilian state, where 

 the fourth and fifth were presumably fully formed and persistent. We 

 may suppose that the genetic factors concerned with the third digit are 

 equipotential with those for the second as far as the eleventh day of 

 incubation, for both digits are then of the same size and each bears 

 two phalanges. Later, the factors for the third digit undergo a com- 

 parative weakening and "ultimately only one or at most two phalanges 

 remain without any claw at the tip, whereas the second digit acquires 

 its three phalanges and a claw. The factors for the third claw have 

 wholly disappeared, while those for the second phalanx are almost lost 

 in some individuals but fully retained in others. 



With the series of embryos available, it would be possible to trace 

 the loss day by day of the vestigial parts of the fourth and third digits 

 as it occurs within the egg. It would merely serve to demonstrate 

 however what is already fully established from the second phalanx of 

 the third, that the reduction and ultimate loss proceed in a gradual, 

 continuous manner. The digits have not disappeared at once in their 



1 "On the early development of the Appendicular Skeleton of the Ostrich, with 

 remarks on the Origin of Birds," Trans. S. A. Pliilos. Soc. Vol. xvi. 1906. 



