THE HAIR 305 



cytoplasmic bridges. The outer layers of cells become cornified, the cornification 

 of the cells proceeding from the stratum germinativum toward the surface. Thus, 

 next the germinal layer are cells containing keratohyalin, which constitute the 

 stratum granulosum, a single layer of ceUs. A thicker layer above the stratum 

 granulosum shows cells in which drops of a substance called eleidin are formed. 

 These droplets, which are supposed to represent softened keratohyalin, give these 

 cells a clear appearance when examined unstained. Hence the layer is termed 

 the stratum lucidum. In the outer layers of the epidermis the thickened walls 

 of the cells become cornified and in the cells themselves a fatty substance collects. 

 These layers of cells constitute the stratum comeum. The cells of this layer are 

 also greatly flattened, especially at the surface. 



When the hairs develop they do not penetrate the outer periderm layer of 

 the epidermis but, as they grow out, lift it off. Hence this layer is known also 

 as the epitrichium (layer upon the hair). Pigment granules appear soon after 

 birth in the cells of the stratum granulosum. These granules are probably formed 

 in situ. Negro children are quite light in color at birth but within six weeks their 

 integument has reached the normal degree of pigmentation. 



The dermis or corium of the integument is developed from mesenchyme or 

 from the dermatomes of the mesodermal segments. For special points concern- 

 ing its development see Keibel and Mall's "Human Embryology," vol. 1, p. 254. 



THE HAIR 



Hairs are derived from thickenings of the epidermis and begin to develop at 

 the end of the second month on the eyebrows, upper lip and chin. The hair of 

 the general body integument appears at the beginning of the fourth month. 



The first evidence of a hair anlage is the elongation of a cluster of epidermal 

 cells in the inner germinal layer (Fig. 293 A). The bases of these cells project into 

 the dermis and, above them, cells of the epidermis are arranged parallel to the 

 surface. The elongated cells continue to grow downward until a cylindrical hair 

 anlage is produced (Fig. 293 B, C). This consists of an outer wall formed of a 

 single layer of columnar cells, continuous with the basal layer of the epidermis. 

 This wall bounds a central mass of irregularly polygonal epidermal cells. About 

 the hair anlage the mesenchyma forms a sheath, and at its base a condensation of 

 mesenchyme produces the anlage of the hair papilla, which projects into the 

 enlarged base of the hair anlage. As development proceeds, the hair anlage grows 

 deeper into the corium and its base enlarges to form the hair bulb (Fig. 293 C). 



