STRUCTURE OF THE VERTEBRATES 



117 



and there are definite regional differences. Hair may be defini- 

 tive in length or continue growing. The latter is usually more 

 marked in the male than the female. There is no definite 

 moulting of hair, though the covering tends to be thicker in win- 

 ter than in summer in animals of cold climates. Hair is con- 

 stantly being replaced in the individual, unless the growth cells 

 at the base are killed, and as one falls it is soon replaced by a 

 new one growing from the bottom of the follicle. 



Each hair has a central medulla with concentric layers of 

 cells around it. The young cells at the base of the hair are 

 cuboidal, but as the hair is pushed upward the cells flatten and 



/ctodermal 

 inpocketing^ 



.Stratum Stratum 



comeum [germinatwum 



Fig. 53. Development of Hair. The four stages represent progressive 

 embrj^onic growth, and are not found in the same specimen. 



die. Those on the outside, the cuticular layer, overlap the more 

 proximal ones and give the hair a scaly appearance. Many 

 species, like the human, have smooth hair. Others have elongated 

 cuticular scales and have a felting qualit}^ like the wool of 

 sheep. Some animals (rabbits and hares for example) have long 



