46 



GENERAL ZOOLOGY 



not function in respiratory processes. It may develop hair 

 (Fig. 22). 



The human epidermis is composed of many layers of cells, 

 morphologically differentiated into distinct regions which, 

 beginning with the lowest or innermost layer, are as follows: (1) 

 stratum germinativum, (2) stratum granulosum, (3) stratum 

 lucidum, and (4) stratum corneum. The basal layer of the 



CO 





Fig. 22. — Vertical section of human skin, semidiagrammatic. b, basal layer 

 of epidermis; c, stratum corneum, the outer layer of epidermis; co, corium; d, 

 duct of sweat gland; e, epidermis; g, stratum granulosum; ge, stratum germi- 

 nativum; h, hair; l, stratum lucidum; s, sweat gland; be, sebaceous gland, open- 

 ing into hair follicle. 



stratum germinativum produces all of the epidermis and from it 

 also develop hair follicles and hair, sweat and oil glands, mam- 

 mary glands, and nails. It is composed of five or six layers of 

 cells, graded in shape from a columnar outline in the basal layer 

 to a flattened polygonal form in the outer layers. The stratum 

 granulosum is about two cell layers in thickness and gets its 

 name from the fact that the cytoplasm of the cells in this layer 

 has undergone a granular degeneration, the product being known 

 as keratohy aline granules. The condition of the cells of the 



