EXISTING ORGANISMS— ANATOMY 



85 



scales, but the two are associated in some mammals and the pat- 

 tern of arrangement persists in some that have lost all trace of 

 scales (Fig. 58). 



Claws, Hoofs, and Nails. These structures are likewise similar 

 in origin (Fig. 59). The first consist of approximately equal con- 

 vex dorsal and concave ventral plates in the more primitive birds 

 and reptiles. Mammalian claws have the ventral plate greatly 

 reduced and the tip of the toe covered ventrad by a terminal pad 

 which is scarcely evident in the other groups. In hoofs the dorsal 



Fig. 59. — Diagrammatic longitudinal sections through digits of various mam- 

 mals to illustrate the morphology of claws, hoofs and nails, a, Echidna, a 

 primitive oviparous mammal; b, a typical clawed mammal (unguiculate); 

 c, horse; d, monkey; e, man. The dorsal plate is in black, ventral plate 

 striped, bones stippled. (From Wilder's Hislorij of the Human Body, a after 

 Gegenliaur, b-e after Boas; with the permission of Mrs. H. H. Wilder and 

 Henry Holt and Company.) 



plate is enlarged and thickened, while the ventral plate is also 

 extensive and horny, though still a rather soft structure. The 

 terminal pad is lacking. In the monkeys nails are found which 

 have a broad dorsal plate extending little if any beyond the tip 

 of the digit; the ventral plate is reduced to a transverse strip 

 beneath the tip of the dorsal plate. In man this reduction is carried 

 still further and the ventral plate is vestigial. The terminal pad 

 is evident in the primates only by the persistent friction ridges 

 with definite pattern which occur on the tips of the digits. 



Vestigial Structures in Man. For complete homologies of the 

 remaining systems, reference must necessarily be made to embryo- 

 logical development, involving greater detail than can be included 

 here, although the resulting comparisons are as conclusive as those 



