50 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



may generally be noticed that the epidermis forms a thin covering over the free 

 toothed end. Draw. 



Both cycloid and ctenoid scales are composed of bone and are homologous with ganoid 

 scales. All three types of scales are produced by heaps of mesenchyme cells of the dermis. The 

 posterior edge of each scale grows outward, carrying the epidermis with it; the latter may re- 

 main as a thin covering over the projecting part of the scale or may be rubbed off. The 

 scales of fishes are thus of dermal origin, with the exception of the placoid scale, which con- 

 tains in addition to its dermal part, a small epidermal contribution. 



E. EXOSKELETON OF AMPHIBIA 



The vast majority of present-day Amphibia have naked skins, that is, an exoskeleton is 

 lacking. A few are provided with minute, concealed dermal scales. The extinct Amphibia 

 (Stegocephala) were commonly clothed in a heavy armor of dermal plates homologous with 

 ganoid scales and similar to them in appearance. 



dermis 



stratum 

 germinativum 



dermis 



A 



B 



FIG. 13. Diagrammatic longitudinal sections through the skin of A, a teleostome fish and B, a 

 reptile, to show the locations of the scales. In A, the scales are in the dermis, while in B they repre- 

 sent thickened portions of the stratum corneum. (From Wiedersheim's Comparative Anatomy of Ver- 

 tebrates, courtesy of the Macmillan Company.) 



F. EXOSKELETON OF REPTILES 



The bodies of reptiles are characteristically clothed with an exoskeleton com- 

 posed of horny scales. These scales are of epidermal origin, representing particu- 

 larly dense, cornified areas of the stratum corneum. In the formation of such a 

 horny scale a dermal papilla first appears which furnishes nutriment for the cells 

 engaged in producing the scale. The stratum germinativum over the dermal papilla 

 begins to proliferate rapidly, producing cells which become flat and horny and 

 compressed into a scale (see Fig. 13^). In addition to the horny scales many 

 reptiles possess bony plates of dermal origin situated beneath the epidermal 

 scales. These bony plates originate in the same manner as the scales of fishes. 

 In order to avoid confusion, the epidermal scales will be designated as scutes and 

 the dermal structures as plates. 



i. Exoskeleton of the lizard. Recall the condition of the lizard, or re- 

 examine the specimen. The body is clothed with horny scales of epidermal 

 origin, overlapping like shingles. Note arrangement and size of the scales on 



