524 



EMBRYOLOGY. 



with the hair-bulb (hz). The cells in the inner sheath of the root 

 (iw) assume a flattened form and become cornified. 



In consequence of the growth which proceeds from the bulb the 

 hairs are gradually shoved up toward the surface of the epidermis, 

 and at the end of the fifth month in the case of Man begin to break 

 forth to the outside (fig. 292 D ha'). They protrude more and 

 more above the surface of the skin, even in the embryo, and consti- 

 tute at many places of the skin, especially on the head, a rather 



Fig. 292 A D, Four diagrams of t:.e development of the hair. A, Development of the hair- 

 papilla on the free surface of the skin, as it occurs, according to GOETTK, in many Mammals. 

 B, C, D. Three different stages of the development of the hair in human embryos. 



ho, Corneous layer of the epidermis ; scltl, mucous layer ; pa, hair-papilla ; hk, germ of hair ; 

 hz, bulb of hair; ha, young hair ; ha', tip of the hair protruding from the hair-follicle ; 

 aw, iw, outer and inner sheath of the root of the hair ; hb, hair-follicle ; id, sebaceous gland 



thick covering. On account of their minute size and fineness, and 

 because they fall out soon after birth, they are called the downy hair 

 or lanugo. 



Each hair is a transitory structure of short duration. After a time 

 it falls out and is replaced by a new one. This process begins even 

 during embryonic life. The hairs that fall off get into the amniotic 

 fluid, and since with this fluid they are swallowed by the embryo, they 

 form one of the components of the. meconium accumulated in the 

 intestinal canal. A more extensive change takes place in Man soon 



