THE EPIDERMIS 265 



various shades of mulatto skin, there is a close correspondence between 

 the degree of color and the number of pigmented layers of epidermal 

 cells. Certain connective tissue cells of the corium of pigmented skins 

 also contain abundant melanin granules; in the darker skins such cells 

 are numerous. However, the dermal pigmented cells are not correctly 

 regarded as the sources of supply of melanic granules for the epidermal 

 cells, as has been maintained. The latter can produce their own 

 granules; both dermal and epidermal pigmented cells owe their condi- 

 tion to the same underlying cause (Jordan; Amer. Nat., vol. 45, 1911). 

 The pigment granules are said to arise as a differentially staining nuclear 

 substance ('pyrenoid substance') which passes through the nuclear mem- 

 brane into the cytoplasm, where it gradually acquires the character of 

 pigment granules (Meirowsky: "On the Origin of the Melanotic Pig- 

 ment in the Skin and the Eye," Leipzig, 1908). The nuclear origin of 

 the pre-pigment granules: is, however, disputed by some. The more re- 

 cent investigations indicate that the physiology of melanic pigment for- 

 mation involves the interaction of a nuclear oxidase (tyrosinase) with 

 an extranuclear, perhaps cytoplasmic, chromogen (ty rosin). 



The processes of mitotic cell division are very active in these 

 columnar cells, and they, with the adjacent portion of the prickle cell 

 layer, form the stratum germinativum of Flemming, in which the re- 

 generation of the epidermis occurs. The cylindrical cells are firmly 

 united to the basement membrane by delicate cytoplasmic fibrils, the 

 intercellular bridges. Their nuclei are ovoid in shape, and vesicular in 

 appearance. 



Prickle Cell Layer (Stratum Spinosum). Superficial to the cylin- 

 drical cells is a stratum of polyhedral epithelium which extends inward 

 between the adjacent papillae of the corium (interpapillary region of 

 the epidermis), and is therefore thick in these portions, but is rela- 

 tively much thinner over the apices of the dermal papillas (suprapa- 

 piUary portion of the epidermis). 



The polyhedral cells of this layer contain a soft granular cytoplasm 

 and a very chromatic, though vesicular, spheroidal nucleus. They are 

 separated from one another by narrow intercellular spaces which are 

 bridged across by innumerable delicate cytoplasmic fibrils. These fibrils 

 connect adjacent cells and are frequently continued without interruption 

 through one, two, or even three or four neighboring cells. Their course 

 is characteristically curved, the convexity being directed toward the 

 nucleus. Those portions of the numerous cytoplasmic fibrilla? which 

 span the intercellular spaces form the so-called uitrrct'lltilar bridges. 



