mammary gland ducts. Lactation hairs may surround the 

 area of the nipple. The position and number of the mam- 

 mary glands varies. 



Reptiles 



The skin of the reptile is covered by keratin scales, each 

 of which covers an epidermal and dermal scale elevation. 

 The scale cover is continuous over the entire body but thin- 

 ner in the grooves between the scales. Periodically it is shed, 

 a process called ecdesis (Figure 8-6). 



Close examination of the lizard's skin shows at least two 

 types of scales; the large scales usually described and smaller 

 ones lying between these (Figure 8-7). In Iguana, the small 

 scales are quite pointed. Histological examination reveals sen- 

 sory structures, the prototrichs, on some large and many small 

 scales (Figures 8-8, 8-9). The protothrix (singular of proto- 

 trichs) of the reptile has been described as the precursor of 

 the hair of the mammal, as the term itself declares. It is quite 

 possible that the small scales margining the large scales are 

 the forerunners of both hair and feathers. The homology of 

 these scales with the feather is most probable. 



Osteoderms sometimes lie in the dermis below the scale. 

 These bony plates may have the same distribution as the 

 keratin scales or be quite independent of them. The bony 

 plates (osteoderms) are tied into the dermis by Sharpey's 

 fibers, just as the scales of fishes are. Osteoderms are typical 

 of the crocodilian and some lizards. In the turtle, the cara- 

 pace is formed basically of osteoderm and rib contributions. 

 These are covered externally by horny epidermal scales; 

 the epidermis lies between these scales and the bony base. 



The epidermis of the reptile is generally without glands, 

 although special glands occur in different reptiles; the lizard 

 has femoral glands along the posteroventral margin of the 

 thigh (Figure 8-6). The alligator has inframandibular and 

 cloacal scent glands which are functional during the repro- 

 ductive period. 



The development of the skin of the reptile is very much 

 the same as that of the mammal (Figure 8-3). At first there 

 is a simple cuboidal ectoderm which differentiates into an 

 outer periderm and a basal germinal layer. As outer layers 



pigment cells 



anterior ^^ 



Figure 8-7. View of on area of scales in the shoulder region 

 of \guana. 



are added, the epidermis becomes stratified, and the scales 

 appear. 



Bird 



The skin of the bird is generally thin since it is overlaid 

 by a thick cover of feathers. The skin is dry with a stratum 



holocrine secretion 



Figure 8-6. Section through skin showing a femoral organ (mole) 

 and the scales of the lizard, tocerto ogi/is. (After Tolg, 1 905) 



Figure 8-8. A, section through a scale of Co/otes with a sensory bristle 

 at its tip. B to D, series of sketches showing transition from wart and 

 prototriches of the amphibion to the retrosquomous hairs of the mam- 

 mal as conceived by Elios and Bortner (1957). 



SKIN 



221 



