INTEGUMENT 



365 



shown resting on a portion of the underlying mesodermal tissues called 

 the derma. This derma is a part of the skin, being especially set apart 

 from the more central 

 tissues and strengthened 

 to afford a strong and 

 elastic bed for the epi- 

 thelium. A basal mem- 

 brane is present, and 

 upon its slightly curved 

 surface rests the basal 

 layer of the epithelium. 

 This layer is made up 

 of a very perfect layer 

 of slightly columnar 

 cells whose nuclei are 

 oval, probably on ac- 

 count of their some- 

 what crowded condition. 

 They lie in the cell at 

 some little distance from 

 the basal membrane, 

 and are frequently met 

 with in mitotic division. 



The many cells de- 

 rived from this basal 

 layer lie above it in a far 

 thicker layer of cells 

 that takes up about one 

 half of the entire thickness of the epidermis. This is known as the 

 stratum germinativum. Most of the cells in this layer show evidences 

 of an amitotic division, a terminal process in the life of the cell. The 

 nucleus and cell both grow in size until, at the outer boundary of the 

 layer, they are transformed by a deposit of granular matter into the 

 cells of a thin granular layer, known as the stratum granulosum. 

 This layer is about two cells thick, and its flattened nuclei show signs of 

 degeneration. The layer stains rather deeply. 



Above this stratum the cells are changed, first into a homogeneous 

 and non-staining stratum lucidum, and then by a stratification and harden- 

 ing as well as flattening, into an outer stratum corneum. Both of these 

 two last layers are dead cells which are toughened and developed to act 

 as buffers between outer abrasion and the delicate and living tissues be- 

 neath them. As fast as any of these cells are rubbed off, new ones are 

 added from beneath. A cell from this outer layer is large and thin and 



FlG. 330. Portion of a vertical section through the epider- 

 mis of man. c., part of the underlying corium; b., basal 

 layer of the epithelium, separated from corium by the thin 

 basement membrane; g., stratum germinativum; gr., 

 stratum granulosum; /., stratum lucidum; cor., stratum 

 corneum. X 350. 



