78 PIGMENT. 



very minute granules, which exhibit a lively molecular motion. 

 This motion may sometimes be observed even within the cells, 

 so that the rest of their contents must be fluid. As it is also 

 known, that the pigment-granules may sometimes be pressed out 

 from the cells, no doubt can exist respecting the cellular nature 

 of these bodies, formerly called pigment-globules. The wall of 

 the pigment-cells exhibits a nucleus, which is already familiar to 

 some observers. It may be seen in the fetal condition of the 

 pigment cells of the choroid coat in mammalia, at different 

 points in that of the very young foetal pig for instance, quite 

 distinctly; and it occasions the well-known white spot in the 

 centre of the cells. It commonly contains one or two nucleoli. 

 It sometimes happens that no pigment-granules are deposited 

 around the nucleus, but that it is surrounded by a clear, trans- 

 parent areola. 



Some pigment-cells undergo a most remarkable transforma- 

 tion, and one which acquires an especial importance, from the 

 fact that it serves as a type of formation for other more im- 

 portant classes of cells. This transformation consists in the 

 cells being elongated on three or more sides into hollow fibres. 

 These we shall name stellated cells. It has, indeed, been 

 necessary to allude to them already when treating of bone. 

 The characteristic contents of the pigment-cells render them 

 best adapted for an accurate examination of this type of for- 

 mation. The stellated pigment-cells, known under the name 

 pigment-ramifications, are best observed in the skin of the 

 tadpole. They exhibit varieties in form ; we select for our 

 description such of them as present the longest fibres. (See plate 

 II, fig. 9.) Their appearance is that of separate black spots, 

 from which slender black fibres issue on different sides. The 

 black spots represent the bodies of the cells filled with pigment ; 

 the fibres are the prolongations of the cells filled with the 

 same material. The separate pigment-granules may be dis- 

 tinguished in many situations. The body of the cell, which is 

 sharply defined on its exterior, sometimes presents a clearer 

 spot of a round or oval form, through which the cell-nucleus 

 glimmers, and in some few instances can be distinctly per- 

 ceived with its nucleolus. The diminution of the cell in various 

 directions, in order to pass over into a fibre, is so gradual that 

 there is no defined limit between them. The fibres pass be- 



