INTEGUMENT 



29 



the point in question 1 (Fig. 22). This thickening of the epiderm 

 grows downwards in the form of a papilla (hair-germ) and 

 is surrounded by the cells of the derm, so that, as in the case 

 of the feather, it comes to lie within a kind of pocket, the hair 

 follicle. The originally uniform mass of cells of the hair-germ is 

 later differentiated into a peripheral and a central portion : the 

 latter (bulb-cone, C) gives rise later (D) to the hair-shaft with its 

 medulla and pith, and to the cortex, as well as to the cuticle of 

 the shaft and the so-called inner root-sheath ; from the former 

 arises the outer root-sheath. The origin of both sheaths, as well 

 as the sebaceous glands, can be traced to the Malpighian layer 

 (cf. Fig. 23.) The base of the hair-shaft which fills up the bottom 

 of the follicle is broadened out to form the hair-bulb, which extends 



A 



B 



D 



FIG. 22. DIAGRAM OF FOUR STAGES IN THE DEVELOPMENT OF THE HAIR 



(founded on Stohr's figures). 



A, hair germ ; B, hair-cone ; C, bulb-cone, showing formation of bulb, papilla, and 

 hair-cone, which latter is becoming cornified at the apex. D, later stage in 

 which the hair is further differentiated, but has not yet reached the surface. 



round the highly vascular hair-papilla like a cap. The hair 

 usually breaks through the skin obliquely, the direction differing 

 in different parts of the body. 



Thus the more or less cylindrical hair-shaft consists of three 

 parts medulla, cortex, and cuticle : the medulla is the most 

 important part of the hair, and on its structure mainly depend 

 the differences seen in the hair of individual species. 



The colour of the hair is due to three causes : firstly, to a 

 greater or less accumulation of pigment in the cortical layer; 

 secondly, to air contained in the intercellular spaces of the 

 medulla ; and lastly to the nature of the surface of the hair, i.e. 

 whether it is rough or smooth. 



The mode of formation of new hairs in post-embryonic stages is 

 not thoroughly understood : when the hair is shed, it is not known 



1 This hair-rudiment at first more or less resembles the rudiment of an 

 integumentary sense-organ of a Fish or gilled Amphibian ; and this fact has led to 

 the expression of a view that the origin of hairs may be traced phylogenetically 

 to such sensor v organs of the lower Vertebrates. 



