AMPHIBIA. 



177 



A superficial epidermis can be distinguished from an underlying 

 dermis, which is considerably thicker. 



The epidermis is made up of several layers of cells, and hence 

 is classified as stratified* epithelium. The most external cells are 

 very flat, without evident nuclei, and make up a horny layer, 

 below which are rounded cells with well-marked nuclei and 

 granular protoplasm. Those abutting upon the dermis, forming 

 the Malpighian layer, are columnar. A small number of irregular 

 pigment cells, containing dark colouring -matter, are scattered 

 through the epidermis. Numerous glands open upon the surface 



Fig. 49. VERTICAL SECTION OF SKIN or FROG (after Wiedersheim), much 

 enlarged. ep, Epidermis; h.l, horny layer; m. I, Malpighian layer; 

 w, warts; gl, cutaneous glands; p.c, pigment cells; De y 1, 2, 3, layers 

 of dermis. 



of the epidermis. They are of two kinds, serous and mucous. The 

 former chiefly occur in the skin of the back, and appear to secrete 



* The chief kinds of Epithelium may be thus classified : 



I. SquamOUS (Pavement). 

 Composed entirely, or mostly, of 

 flattened, cubical, or polyhedral 

 cells. 



II. Columnar. Composed of 



cells more or less elongated at 

 right angles to surface. May be 

 ciliated. 



III. Glandular. Composed 



of spherical or cubical cells, 

 which elaborate a secretion or 

 excretion. 

 9 



1. Simple. One cell thick. Endo- 

 thelium is a variety of this, composed of 

 flattened cells, and lining heart, vessels, 

 body-cavity, &c. 



2. Stratified. More than one cell 

 thick. Epidermis, &c. 



1. Simple. Stomach and intestine. 



2. Stratified. Mouth-cavity of frog. 



For the most part Simple. Peptic 

 glands, kidney tubules, &c., &c. 



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