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 foetal 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, transparent areola.
Some pigment-cells undergo a most remarkable transformation, and one which acquires an especial importance, from the fact that it serves as a type of formation for other more important 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 distinguished 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 perceived 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-
