328 



COLLEGE ZOOLOGY 



Stratum corneum 



Stratum germinativum — i^ 

 Pigment cells 



Epidermis 

 Mucous gland 



Stratum spongiosum 

 Poison gland 



Stratum compactum 



Nerve 



Subcutaneous tissue- 



Dermis 



.- .. . .'...^is!?' ....' ■••;.•.•.•.....":.... ..'^ ■■■•••.. v!v' 



^V/-'-'"-''-:'.::7^--.^^-'i-:.-"" 



Blood cell 

 ■Blood vessel 



Figure 210. Section of frog skin showing microscopic structure (histology). Vertical section. 

 (After Laboratory Explorations in General Zoology, by Karl A. Stiles. Third edition. Copyright 

 1955 by The Macmillan Company.) 



the dorsal and lateral surfaces is darker than 

 the ventral surface, which is whitish. 

 Changes in color may occur as described on 

 page 398. 



Histologically, the skin consists of two 

 layers as in other vertebrates (Fig. 210), an 

 outer epidermis and an inner dermis. There 

 are several layers of cells in the epidermis 

 (stratified epithelium, Fig. 43). That on the 

 outside, the stratum corneum, is horny and 

 consists of broad, thin cells, the squamous 

 epithelium. Beneath this lies a transitional 

 zone of polygonal cells, which in turn rest 

 upon the basal columnar cells of the stratum 

 germinativum. During molting the stratum 

 corneum is shed. New cells are being con- 

 tinually formed by those of the columnar 

 layer, and the outward pressure of these 

 cells brings about the flattening of the 

 surface layers. 



The dermis consists of two layers, a loose 

 outer layer, the stratum spongiosum, which 

 contains the dermal glands and pigment 

 cells (chromatophores), and a dense inner 

 layer of connective tissue, the stratum com- 

 pactum, which contains white and yellow 

 fibers, a few smooth muscle cells, blood ves- 

 sels, and nerves. Beneath the dermis is a 

 subcutaneous layer of loose connective tis- 

 sue, which is divided into two layers by 



lymph spaces and serves to attach the skin 

 to the body wall by septa. 



The skin is richly provided with glands. 

 These are of two principal types: mucous 

 glands, which are more numerous and small, 

 and poison glands, which are larger and less 

 common. These glands open to the outside 

 bv means of ducts. The mucus-covered sur- 

 face of the frog makes him slippery, which 

 often helps him to escape the grasp of 

 enemies. Each gland consists of an epithelial 

 layer of secreting cells, outside of which are 

 muscle fibers and connective tissue. The 

 mucous glands may be present in great num- 

 bers, as many as 60 to each square millimeter 

 of surface. Mucus is formed in the secreting 

 epithelium, discharged into the lumen of 

 the gland, and forced through the duct to 

 the surface of the skin by the muscle cells. 

 The poison glands secrete a whitish fluid 

 with a burning taste, which serves as a 

 means of protection against enemies. The 

 expulsion of this secretion may be stimulated 

 by rough handling or chloroforming a frog. 



The skin in man and 

 other vertebrates 



The skin in vertebrates, in general, is 

 chiefly protective and sensory, but it may 



