DEVELOPMENT OF ANI 311 



digit. They are scarcely noticeable in the adult, but, if we 

 examine some of the earlier stages in the growth of the wing, 

 we find them large and e^^idently playing a part in the life 

 cycle. 



The first sign of any protuberance is found in the early 

 embryo when it is about two-fifths developed. They take 

 the form of a fleshy hook on the poUux and a fleshy hook-like 

 knob at the tip of the wing. There is no sign of an extra 

 phalanx at either place. As the embryonic development 

 advances the hook on the pollux becomes blunter and round- 

 er until, twenty-four hours before hatching, it is a large, 

 rounded, fleshy knob covering the whole tip, though mainly 

 on the ventral side. (Fig. 104a and b.) The knob on the 

 second digit, also mainly on the under side, is larger and 

 rounder and covers the tip. Both growths have hardened 

 and become calloused. After hatching they grow smaller 

 and at the same time harden until, at the time the bird is 

 ready to leave the nest, they are very small and almost bony. 

 (Fig. 104c.) 



The fact tliat the knobs are entirely dermal, though of 

 claw-like appearance in the young embryo, shows that they 

 are a later specialization of what in former times probably 

 were well developed and functional claws. The egg shell of 

 the ani is of very great thickness and the egg-tooth of the 

 embryo is comparatively small. Therefore, after the shell 

 has been cracked, some comparatively strong force must be 

 brought to bear upon it to pry it apart for the escape of 

 the little chick. Consequently tlie wings and feet must be 

 used for this pm-pose after the manner of hatching chicks 

 of the domestic fowl. But, in this case, the shell being much 

 thicker proportionally, the wings are especially e(juipped for 

 pushing and prying and are undoubtedly of much use in 

 this firjst great effort. This is borne out by the fact that the 

 knobs are larger and much more developed at the time of 

 hatching than either before or after. 



