84 HISTORY OF THE HUMAN BODY 



ians (Gymnophiona), in which scale rudiments lie in pits sunk 

 beneath the surface. In the extinct group of Stegocephali, 

 which possessed many amphibian characters, the body was 

 covered ventrally with well-developed, imbricated scales. These 

 facts together furnish sufficient proof that amphibians were 

 originally scaly and that the present naked condition is due to 

 a secondary reduction. These scales were probably bony, like 

 those of ganoids and teleosts. 



It is an abrupt transition from the scales of fishes to those of 

 reptiles, since, in this latter class, the scales are purely epi- 

 dermic in origin and are composed of horn (keratin), a sub- 

 stance allied to enamel, without trace of bone. The corium, 

 it is true, nourishes the scales by means of richly vascular 

 papillae placed beneath each, but furnishes none of the hard 

 parts. There is no doubt that in some way these scales must 

 be related to the bony ones of ganoids and teleosts, but the 

 relation appears to be an indirect one. They may have had 

 a common origin in scales which, like those of the placoid 

 type, possess both elements, the one emphasizing the epidermic 

 portion, the other that of the corium. This would seem to con- 

 flict with the direct derivation of reptiles from the ganoids 

 as we know them, and shows the incompleteness of our records 

 at this point. 



Aside from scales the reptilian integument possesses a great 

 variety of other exoskeletal forms, such as spines, combs, and 

 claws, all made of keratin, and equally unlike anything in 

 ganoids or amphibians. In this wealth of horny exoskeletal 

 elements the reptiles are closely followed by their lineal de- 

 scendants, the birds, where the scales are represented by the 

 far more elaborate, but strictly homologous, feathers, and 

 where beak and feet are encased in horny coverings. The cov- 

 ering for the beak has evidently replaced teeth, as in turtles, 

 and is undoubtedly a recently acquired character, since fossil 

 birds occur in the Cretaceous formation, in all respects like 

 modern birds save in the presence of conical teeth set in 

 sockets ; furthermore, tooth germs have been found in the jaws 

 of the embryos of several species of modern birds, transitory 



