DEVELOPMKM' OF AM ;{ii 



(li,i>il. Tliey arc scarcely noticeable in the adult, htit. if we 

 examine some of the eai'lier sta^'es in the <4i-()\\th of the win^', 

 we find them lar<>'e and evidently playing a pait in the life 

 cycle . 



The first sign of any protnherance is found in the earlv 

 emhi-yo \vhen it is about t\\()-lil'ths developed. They take 

 the Tonn of a fleshy hook on the pollux and a fleshy hook-like 

 knob at the lip oi' the wing. There is no sign of an extra 

 j)halanx at either place. As the embryonic (]evelo])ment 

 advances the hook on the pollux becomes blunter and round- 

 el- ujitil, twenty-four hours before hatching, it is a lai-ge, 

 i-ounded, fleshy knob covering the whole ti]), though mainly 

 on the ventral side. (Fig. lOki and b.) 'I'he knob on the 

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

 rounder and covers tlie 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 that the knobs are entirely dernial. though of 

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

 are a later specialization of what in former times probably 

 were well develoj^ed and functional claws. The egg shell of 

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

 embryo is comparatively small. 'J'herefoi'e, after the shell 

 has been cracked, some comparatively strong force must be 

 brought to bear upon it to pry it apart foi- the escape of 

 the little chick. Consequently the wings and feet must be 

 used for this piu'pose after the manner of hatching chicks 

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

 thicker proportionally, the w'ings are especially equipped for 

 pushing and prying and are undoubtedly of much use in 

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

 knobs are larger and much more developed at tlie time of 

 hatching than either before or after. 



