PIGMENT.
tween the cells of the epithelium, and are therefore frequently curved : they are in general thickest in the neighbourhood of the cell, and diminish as they proceed from it ; but they sometimes also swell out slightly at some distance from the cell. The presence These fibres give off others at different points. of the cell-nucleus, and the fact that all the stages of transition from indubitable pigment-cells to these bodies may be demonstrated, are sufficient evidence that these black spots, with the fibres proceeding from them, are actually cells, and that the fibres are hollow prolongations of them filled with pigment. These transitions are delineated in plate II, fig. 8, just as they existed close together in another part of the tail of a tadpole: a is an indubitable pigment-cell, scarcely differing from an The only circumstance ordinary one; it has also its nucleus. which distinguishes the majority of the primitive cells of these stellated pigment-cells from common pigment-cells is that they are generally smaller, and more closely filled with pigment. b is a smaller cell, which has commenced to taper; and c is distinctly elongated into a fibre. A slightly clearer spot is the d and e only indication of the nucleus in both instances. elongate at both ends into fibres, one of which (the upper end of d) terminates in a knob witha defined outline. At the spot where this knob unites with the body of the cell, a shading, indicating a cavity, may be clearly perceived, the pigment being more closely deposited in the neighbourhood of the cell wall than in the centre; and lastly, f is a cell which elongates into fibres on three sides. When a small piece of the skin of the tadpole is torn in water, separate portions of these pigment-fibres, or prolongations of the cells filled with pigment, may be observed to float about isolated. Instances sometimes occur in which one of these pigment-fibres passes uninterruptedly from the body of one cell to that of another; for example, fig. 9, a. We may imagine this to be effected by the prolongations of two cells meeting at one point. As the pigment does not move from one cell to another, we cannot accurately determine whether the partition-walls become absorbed at such a point or not. Such, however, may be supposed to be the case, otherwise an interruption of the pigment corresponding to the double thickness of the cell-wall must be seen at the spot where the prolongations are in
