272 ESSENTIALS OF ZOOLOGY 



protective envelope to the body, and in the case of mammals 

 it is characteristically aided in this by the development of hair. 

 Hairs are developed as downgrowths of the ectoderm, into 

 the lower end of which a plug of the dermis is received. It is 

 at first solid, but becomes differentiated into a follicle and the 

 hair. The hair then commences to elongate within the follicle 

 due to the multiplication of cells at the base or bulb, and 

 continues to elongate after it reaches the surface. Associated 

 with the hairs are glands, sebaceous glands and sweat glands. 



Epidermis 



Dermis 



Enamel organ 



Dental germ or 

 dermal pulp 



FIG. 133. A section showing an early stage in the development 

 of a molar tooth. 



The former are generally present and distributed all over the 

 skin, but the sweat glands have usually a restricted distribution. 

 The skin also gives rise to mammary glands, but the relation- 

 ship of the mammary, glands to the other two kinds of skin 

 glands is not yet clear. 



The epidermis becomes especially modified to form the 

 claws, which grow in each case from their base, and it is from 

 the skin of the mouth that the teeth are produced. During 

 the formation of the gums a dental ridge is formed. From 

 this downgrowths take place at intervals to form the teeth. 

 Each of these expands as the enamel organ. The inner part 

 of this is indented by a dermal pulp. The inner layer of the 

 enamel organ gives rise to the enamel of the tooth, and the 



