180 THE BIOLOGY OF THE FROG chap. 



The epidermis, or outer portion of the skin, is composed 

 of several layers of cells. The cells of the innermost layer 

 are columnar ; but in passing toward the outer surface the 

 cells become more and more flattened, until those of the 

 outermost or horny layer {stratum corneuni) become very 

 broad and thin. It is the stratum corneum that is shed during 

 the molting process. The gradual change in shape between 

 the cells of the inner and outer surfaces of the epidermis 

 is due to the fact that there is a continual production of new 

 cells in the inner layer which are gradually pushed outward, 

 becoming more and more flattened the farther they are 

 pressed away from their point of origin. 



The epidermis, especially on the dorsal side of the body, 

 usually contains more or less dark brown or black pigment. 

 This pigment is partly within special cells, the chromato- 

 phores, and partly in and between the typical cells of the 

 epidermis. In certain regions all of the cells of the epi- 

 dermis may contain small pigment granules. Ermann found 

 that in the same region of epidermis pigment would appear 

 and disappear in the course of a few months. The chro- 

 matophores of the epidermis resemble the dark pigment 

 cells of the corium. Whether they are derived from cells 

 of the corium which have wandered into the epidermis, or 

 whether they arise through the transformation of cells of the 

 epidermis itself, is a matter of controversy. Loeb and Strong l 

 have come to the conclusion that the chromatophores that 

 appear in the regenerated epithelium of the frog are derived 

 from epithelial cells, and not from cells that have wandered 

 in from the cutis. Chromatophores in the epidermis are 

 not usually abundant. The main source of the color of the 

 skin is in the pigment cells of the corium. 



The inner layer of the epidermis contains several stelhte 

 1 Loeb and Strong, Am. Jour. Anat., Vol. 3, p. 275, 1904. 



