128 THE FROG chap 



The skin is clearly divisible into two layers, an outer, 

 the epiderni (Fig. 38, E) and an inner, the derm (D). The 

 epiderm is built up of several layers of epithelial cells. 

 These differ greatly in form according to their position, 

 those in the lower or internal layer {m. t) being columnar, 

 while those in the upper or external layer {h. t) are squamous, 

 and have their protoplasm converted into horny matter so 

 as to furnish a comparatively hard and insensitive covering 

 to the body. 



The horny layer is cast off periodically in shreds, and to 

 make up for this, the cells of the inner or deep layer 

 multiply by binary fission, the increase in their number 

 necessarily resulting in a pushing upwards of the super- 

 jacent layers. There is thus a constant travelling of cells 

 from the inner to the outer surface of the epiderm : as 

 they pass towards the outer surface they become more and 

 more flattened, and at last squamous and horny. The 

 whole process takes place in such a way that the multi- 

 plication of the columnar cells in the lower layer is just 

 sufiScient to make good the loss of the squamous cells in the 

 superficial layer. 



The derm (D) is formed of connective tissue, the fibres of 

 which are mostly horizontal (h.f, h.f ', h.f"), or parallel to the 

 surface of the skin, but at intervals are found bands of 

 vertical fibres (»./)• The derm also differs from the 

 epiderm in having an abundant blood-supply {b. v), capillaries 

 ramifying through it in all directions. It also contains 

 nerves, the ultimate fibres of which have been traced into 

 the deeper layers of the epiderm. Imbedded in the derm, 

 especially in its external portion, are irregular cells {/>g), the 

 protoplasm of which contains an intensely black pigment. It 

 is to these pigment-cells, already referred to on p. 123, that 

 the black patches in the frog's skin are due. 



