THE INTEGUMENT AND THE EXOSKELETON 107 



and covered with friction-ridges. In monkeys (d) the dorsal 

 plate is flatter and broader as an adaptation to the prehensile 

 hand or foot and does not project much beyond the end of the 

 digit; the ventral plate is much reduced in extent and is not 

 very horny, and the terminal pad has decreased in volume and 

 is indicated mainly by the friction-ridges, which are in the 

 form of a loop or whorl. The extreme of this line of develop- 



FIG. 28. Diagrammatic longitudinal sections through digits of various 

 mammals, to illustrate the morphology of claws, hoofs, and nails, [(a), 

 after GEGENBAUR; (b)-(e), after BOAS.] 



(a) Echidna, (b) Typical unguiculate. (c) Horse, (d) Monkey, (e) Man. 

 The dorsal plate is represented by solid black; the ventral plate is striped. The 

 bones are dotted. 



ment is reached by man (e) in which the last remnant of the 

 ventral plate appears in the narrow strip of skin between the 

 inner surface of the nail (i e., the dorsal plate) and a terminal 

 fold where the friction-ridges commence. The terminal pad 

 is much as in monkeys. In the hoofed quadruped another line 

 of development is shown (c) in which the ventral plate forms 

 a horny, though rather soft, surface for contact with the 

 ground. There is no pad, and the soft integument represent- 

 ing that area lies behind the hoof, continuous with the ventral 

 plate. 



Glands occur in the integument of all vertebrates, profusely 

 in fishes, amphibians and mammals, rarely and strictly local- 

 ized in the sauropsida. They are always derived from the 

 stratum germinativum of the epidermis and vary greatly in 



