horny outer layer 



^ granular layer 



Malpighion or germinative layer 

 ■basement membrane 



capillary 



bone 



Figure 8-1 1 . Section through the bony sheath of the bill of o young 

 sparrow. (After Witschi and Woods, 1936) 



quently slimy. The frog skin, representative of the class, has 

 a low, columnar stratum germinativum, several layers of 

 increasingly shorter cells grading into flattened cells, and an 

 outer squamous ("scaly") layer, which is slightly cornified. 

 In toads the stratum corneum is thicker. In most amphi- 

 bians a periodic ecdesis of the stratum corneum occurs. 

 Keratinization is carried further in forming the covers of 

 the tips of the toes and in the digging spade of Petobates, the 

 Spade-foot Toad. Most amphibians have microscopic bumps 

 or warts on the skin which are the sensory papillae and are 

 formed of knots of enlarged cells associated with nerve end- 

 ings (Fi,gure 8-13). The sensory papilla may have a thickened 

 overlying stratum corneum suggesting the protothri.x. 



mucous gland 



pigment cells 



Figure 8-12. Somewhat diagrammatic section of frog skin (Rona 

 pip/ens). 



sensory endings 



Figure 8-13. Prototriches of the skin of amphibians. A, surface view 

 of skin of toad, Scoph/opus hammondi; B, section of sensory bud of 

 Cryptobranchus; C, section of circumfossate cone of Scop/iiopus 

 hurteri; D, section through cone of Bufo amerjcano. (After Elias and 

 Bortner, 1957) 



The epidermis is separated from the dermis by a base- 

 ment membrane and a fibrous layer containing pigment 

 cells. There is a problem here of differentiating between 

 cells which actually produce the pigment, chromatoblasts, 

 and cells, the chromatophores, which contain pigment ac- 

 quired from the chromatoblasts. The chromatoblasts are of 

 neural crest origin (Figures 7-7 J and 9-15) and have nu- 

 merous and irregular processes extending out from the cell 

 body. Cells with such processes will hereafter be referred to 

 as chromatocytes. The chromatocytes act in the color 

 changes so characteristic of amphibians and reptiles. Some 

 of the chromatocytes invade the epidermis. 



Embedded in the dermis are saccular poison and mucous 

 glands of epidermal origin. The poison gland, with granular 

 secretory cells, is the larger and more deeply located. The 

 cells of the poison gland wall are irregularly spaced with 

 clumps of nuclei around the periphery. This type of gland 

 has a heavier sheath of connective and muscular tissue. 

 The poison glands occur on the dorsal surface of the body 

 and function as a predation deterrent. 



The mucous glands, lying close below the epidermis, 

 have a low cuboidal, or almost squamous, cell lining. Oc- 

 casionally, mixed mucous and poison glands are observed. 



In Necturus, there are numerous club or flask-shaped cells 

 in the epidermis (Figure 8-14). These cells appear to be 



SKIN 



223 



