82 



EVOLUTION AND GENETICS 



tion of phalanges and others shortened, even to a rudimentary 

 state. 



The Exoskeleton. The integumentary structures of verte- 

 brates inckide, in addition to the skin itself, glands, claws, hoofs, 

 nails, horns, teeth, scales, feathers and hair, among which several 

 homologies are evident. 



Placoid Scales. If we return again to the elasmobranchs, we 

 find that their skin is studded with minute scales of a type quite 



a V 1) 



1-^ V 1 1 H 1 1 1 1 W 



''■•^m 



Fig. 57. — Comparison of the development and structure of a placoid scale 

 and a tooth, a, b, and c represent the scale; d, e, and f represent the tooth. 

 The corneous layer of the epidermis is dotted, the germinative layer is 

 represented as a single layer of large cells with nuclei. The dermal layer of 

 the skin is represented as a filirous layer with scattered cells, x, enamel- 

 producing cells; y, mesodermal papilla; e, enamel; d, dentine; p, pulp cavity. 

 (From Wilder's Histori/ of the Human Body, with the permission of Mrs. 

 H. H. Wilder and Henry Holt and Company.) 



different from those of other fishes. These are called placoid 

 scales, and consist of a flattened base formed of dentine, a fine, 

 compact bony substance, from which projects an oblique cusp. 

 The whole is covered by a layer of enamel which is thickest at the 

 tip of the cusp. A mesodermal papilla projects into the hollow 

 under surface of the scale and furnishes it nourishment. A study 

 of the development of these scales discloses that the dentinal 

 portion is derived from the inner, or dermal layer, of the skin, while 

 the enamel is formed from the enveloping epidermal layer. 



Scales and Teeth. The jaws of these fishes are armed with 

 numerous teeth arranged in rows, which bear a conspicuous re- 



