n8 HISTORY OF THE HUMAN BODY 



time an exclusive characteristic. Since neither natural selec- 

 tion nor sexual selection could have a part in this transmis- 

 sion, it has been brought forward as a case of the direct trans- 

 mission of an acquired characteristic. It may, however, be 

 a phenomenon similar to the strange series of homologies 

 of the various parts of the reproductive organs, treated in full 

 in Chapter IX., where such an explanation is inadmissable, 

 although at the present time no satisfactory one can be offered. 

 Although usually the only parts of the mammary apparatus 

 to occur in the male are the rudimentary nipples, yet cases of 

 so-called gynecomastism are known, in which well-defined and 

 even functional mammae occur in persons of the male sex, 

 unaccompanied by any sexual abnormality. 



Pigment is a coloring matter, occurring in the form of 

 granules, and existing in certain cells as a form of metaplasm 

 secreted by them and retained within their substance. These 

 pigment cells are found in both epithelium and connective 

 tissue. Pigment often occurs in the interior of the body, 

 notably in the peritoneum of amphibians, where it lines the 

 ccelom and invests the organs with a brown or even black 

 covering. It occurs in the integument or the integumental 

 structures of all vertebrates except certain white animals, and 

 in albinos, the peculiarity of which consists of a total absence 

 of pigment from all parts of the body. These two cases may 

 be readily distinguished by observing the iris of the eye, which 

 retains its pigment in normally white animals, but lacks it in 

 albinos, giving the eyes a pinkish cast. As a general rule 

 the integument of vertebrates is pigmented when without ac- 

 cessory structures, or when these form an insufficient covering, 

 but in those birds and mammals in which the feathers or hair 

 are respectively sufficient to entirely conceal the skin, these ac- 

 cessory parts receive the color and the integument is unpig- 

 mented. This rule is further emphasized by the fact that bare 

 places, like the head and neck of vultures and the ischial cal- 

 losities of monkeys, or scantily covered places like the entire 

 integument of elephants and rhinoceroses, are pigmented, and 

 occasionally highly colored. 



