CHORDATA 



327 



may be purely epidermal, as in hair, feathers, nails^ hoofs, claws, 

 and the scales of some reptiles ; or in other instances the principal 

 structures are formed in the dermis, usually with an outer layer 



Fig. 155- Diagram of the skin in Mammals, showing the multiple layered condition, together 

 with outgrowths and ingrowths. Drawn by Dr. J. W. Folsom. E, epidermis; D, dermis; a, adi- 

 pose tissue, — fat deposited amid the connective tissue; b, blood vessels; c.e., columnar epithelial 

 layer of the epidermis;/, hair follicle; h, hair; n, nerve; n.e., nerve ending (sensory corpuscle); p, 

 pore of sweat gland; seb., sebaceous or oil gland of hair; s.g., sweat gland; s.c, horny layer of epider- 

 mis; s.m., mucous layer of epidermis. 



Questions on the figure. — What suggests that the columnar layer of the epi- 

 dermis is the most vitally important layer? Are the hair and glands dermal or 

 epidermal growths? Which structures found in the dermis seem to be invasions 

 of that layer by outside structures? What are the functions of the various layers 

 of the skin? Which parts are ectodermal and which mesodermal in origin? 



contributed by the epidermis, as in the teeth or the scales and 

 bony plates which form in many instances (turtle, armadillo, 

 etc.), a very complete external skeleton. Some of the bones 



