128 THE FROG CHAP. 



the epiderm (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. I) being 

 columnar, while those in the upper or external layer (h. I) 

 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 

 multiplication of the columnar cells in the lower layer is 

 just sufficient 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 (v. f). 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 (pg), the proto- 

 plasm of which contains pigment, often appearing 

 intensely black. It is to these pigment-cells that the 

 coloured patches in the frog's skin are due (Figs. 24 

 and 27). 



In this as well as in the other sections described in 



