396 



VERTEBRATE LIFE AND ORGANIZATION 



Strat u. ni 



corneu.m. 

 Slra-tum 



Oermina.tivuin 

 Chrotnatophora 



Mucous ^lancL 



Blood ve-ssal 



-Epidermis 



>- Dermis 



■ Nerve. 1 



• .^ 



Figure 21.2. A photomicrograph of a vertical section through the skin of a frog. 



The dermis consists of fibrous connective tissue. The fibers in the 

 deep portion are more regularly arranged and more tightly packed than 

 those immediately beneath the epidermis. The deeper layer of the 

 dermis, which commonly contains a few smooth muscle fibers, constitutes 

 the stratum compactum, whereas the more superficial layer is known 

 as the stratum spongiosum. Blood vessels, nerves and simple sense organs 

 are found throughout the dermis. They come close to the epidermis, but 

 only a few naked nerve processes actually enter this layer. 



The stratum spongiosum contains many alveolar glands, which 

 consist of simple, round sacs of cells that have pushed into the dermis 

 from the epidermis. They have an epithelial wall and a cavity or lumen 

 which remains connected to the surface by a duct. The most numerous 

 glands are mucous glands, whose secretion is a slimy mucus that is 

 discharged over the surface of the body where it helps to protect the frog 

 against desiccation and excessive water entrance. A few poison glands 

 are found in certain areas of the skin, notably in the dorsolateral folds 

 in Rana pipiens. These are larger and produce a watery secretion that 

 is presumed to be distasteful and irritating to certain of the frog's 

 predators. 



Frog skin is richly colored. In Raiia pipiens, the general greenish 

 tone blends with the surroundings, while the darker spots and blotches 

 tend to obscure the form of the animal. This concealing coloration 

 presumably helps the frog elude its predators and stalk its prey. 



Most of the pigment and refractive granules responsible for the 

 coloration are contained within stellate cells known as chromatophores, 

 which are concentrated just beneath the epidermis. Some chromato- 

 phores (melanophores) contain a brown to blackish pigment, some 

 (lipophores) a yellowish to reddish pigment, and some (guanophores) 

 refractive granules of guanine. There is no green pigment in frog skin. 



