THE SKIN. 213 



the epidermis. Thus vvc see that when the automatic action of the 

 skin is inadequate for the performance of its functions it calls forth 

 an auxiliary activity of other organs through the medium of the 

 nervous system. 



The hair-follicles are developed from the rete mucosum of the epi- 

 dermis, and first appear as little masses of cells growing into the 

 underlying connective tissues (Fig. 189). The sebaceous glands 

 arise as offshoots from these cellular masses. 



Fig. 191. 



L 



-B 



--Sr 



• ' 



Section of developing tooth. From embryo of rabbit. (Freuiid.) ep, epithelium of gum: 

 sit, epithelial cells forming outer layer of the enamel-pulp of the temporary tooth ; L, sim- 

 ilar layer belonging to the rudiment of the permanent tooth ; Sr, enamel-pulp; p, dental 

 pulp of the tooth-cavity ; d, dentin ; v, bloodvessels ; B, rudiment of second or permanent 

 tooth ; a, embryonic connective tissue of the alveolar process. 



The Teeth. — The development of the teeth presents close anal- 

 ogies to that of the hairs. They also first appear as little masses 

 of cells, growing into the connective tissues of the alveolar proc- 

 esses from the stratified epithelium covering them. Into the bases 

 of these masses connective-tissue papilla? are developed, which 

 eventually become differentiated into the pulp of the tooth-cavities. 

 The epithelial cells which immediately surround these papilla? be- 

 come elongated to a columnar form and then become converted 

 into or elaborate the tissue of the enamel. The superficial cells 



