172 



COMPARATIVE ANATOMY 



Both have their bases surrounded by a fold of skin, the "nail wall." 

 In both, a convex outer plate on the upper side of the digit may be dis- 

 tinguished from a concave "ventral plate" on the under side, each being 

 morphologically a reptilian scale. The ventral plate in man is reduced 

 to a narrow fold of skin between the nail and the finger pad. 



Claws appear first in urodele amphibia. In some Anura, they are 

 limited to certain hind toes. But certain male frogs, at mating time, 

 develop horny papillae on their thumbs, which serve to hold a slippery 

 female. Reptiles have claws on all toes. Those of mammals are like 

 those of reptiles, except where the mammalian claw has altered into a 

 hoof, or become retractile, as in cats, which walk on foot-pads and keep 

 their claws sharp by raising them off the ground. Claws of mammals 

 intergrade with nails, so that it is difficult to draw a line between the two. 



Fig. 130. — Diagrams of (A) nails, {B) claws, and (C) hoofs, e, unmodified epidermis; 

 M, unguis; 5, subunguis. (From Kingsley, after Boas.) 



Some primates have both claws and nails on the same foot. Nails are 

 then rudimentary claws, modified to correlate with the increased sensi- 

 bility of the ends of the digits and their use as organs of touch. 



Some mammals, such as the horse and deer, which run on their toes, 

 have hoofs instead of claws. The structure, development, and relations 

 of hoofs, however, prove that they are nothing more than enlarged and 

 modified claws. Both have dorsal and ventral plates. The attempt to 

 divide mammals into hoofed and clawed tj^jes encounters the difficulty 

 that at least one animal, Hyrax, has both claws and hoofs. 



FEATHERS 



Feathers, which are characteristic of birds, are modified scales, and 

 their early development is the same. A corium papilla initiates both; 

 but the feather anlage, instead of flattening to a scale, becomes an elon- 

 gated cylinder, which spHts into the plumes, barbs, and barbules of the 

 developed feather. A down feather, in fact, suggests an elongated and 

 frayed out scale. 



