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The overall pattern of hair follicle arrangement in the rat and mouse.

The presence of a hairy integument is one of the fundamental characteristics of all mammals, except for the almost completely hairless skin of whales and dolphins (Danforth, 1925; Giacometti, 1967). In the past, different aspects of hair and hair follicle structure have been described. They include (a) the arrangement of hair follicles in groups in different mammals (de Meijere, 1894; Dawson, 1930; Fraser, 1929, 1931; Duerden, 1939; Carter, 1943a; Noback, 1951; Oberste-Lehn & Nobis, 1963), (b) the density of hair in different species, and hair density on different areas of the body in the same animal (Dry, 1926; Schultz, 1931; Carter, 1943b; Szabo, 1958), (c) the pattern of hair tracts or the direction of hairs on the body surface (Waldeyer, 1884; Kidd, 1903; Landauer, 1925; Wood Jones, 1941; Kiil, 1949), and (d) a classification of the different types of hairs and follicles (Friedenthal, 1908, 1911; Dry, 1926; Straile, 1958, 1960; Whiteley, 1958; Greene, 1959; Kawamura, Nishiyama, Ikeda & Tajima, 1964). In the integument of the rat and mouse, there are present, apart from the vibrissae and the short woolly hairs of the undercoat, long guard-hairs called monotrichs by Dry (1926). The latter have been referred to by Straile (1958, 1960) as tylotrichs, which are closely associated with the 'Haarscheibe' of Pinkus (1927). (See discussion below and also Fig. 1.) The commonest, but not universal, grouping of follicles in the animals studied is apparently the trio, in which a central large hair follicle is flanked on either side by two smaller hair follicles. Groups of up to about a dozen separate follicles are also known to occur. Compound hair follicles, the 'echte Haarbundel' of de Meijere (1894), are present in some animals. In the skin of the adult chinchilla, for example, up to seventy-five hairs are described emerging from a single common pore (Wilcox, 1950). The fact that groups of hair follicles lie in rows has also been known (de Meijere, 1894; Dawson, 1930; Trotter, 1932), but there is no description of the linear or other arrangement in all parts of the skin of any single type of animal. This paper describes the overall pattern ofarrangement of hair follicles in the complete skin of the common laboratory rat and mouse.