INTEGUMENT OF MAMMALS 



141 



papillfie. Since the outer surface of the epidermis is smooth, its di- 

 ameter must be thicker at some places than at others. The stratum 

 germinati\um, or Mal])ighian layer, consists of a base of short colum- 

 nar cells ()\erlying which are several strata of polyhedral cells, the 

 outermost being somewhat flatter than the inner. Frequent cell di- 

 visions occur in the cells of the Malpighian layer, and at each division 

 one of the daughter cells is pushed out toward the surface. Since 

 this process is continuous, older cells are constantly being pushed 

 further and fiu-ther away from the basal layer. Consequently, 

 each cell passes through the three outer zones and is eventually 

 cast off from the surface. So-called protoplasmic bridges connect 

 adjacent cells of the stratum germinati\'um and have led some to 

 call them "prickle" cells. Skin color depends on dift'erent colored 



Fig. 85. — .Section through the foot-pad of a squirrel, showing four regions of epidermis. 



pigment granules in the lower cells of the stratum germinativum. 

 These pigments range through black to yellow and red and may be 

 present only in the lower layer of cells, or may extend to all the cells 

 of the stratum germinativum. 



Stratum Granulosum. — Thh is a zone of several layers of cells 

 outside the stratum germinativum. The cells have a flattened poly- 

 hedral shape. Irregular-sized granules which are composed of a 

 substance called keratohyaline represent some definite chemical 

 change in the protoplasm of these cells. The nuclei of the outer 

 cells of this granular zone disintegrate. 



Stratum Lucidum. — This layer appears as a bright, clear band 

 just above the stratum granulosum and consists of closely packed 



