PIGMENT. 79 



tween tlic cells of the epithelium, and are therefore frequently 

 curved : they arc in general thickest in the neighbourhood of 

 the cell, and diminish as they proceed from it ; but they some- 

 times also swell out slightly at some distance from the cell. 

 These fibres give oft* others at different points. The presence 

 of the cell-nuclenSj and the fact that all the stages of transition 

 from indubitable pigment-cells to these bodies may be demon- 

 strated, arc sufficient evidence that these black spots, with the 

 fibres proceeding from them, are actually cells, and that th 

 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 

 ordinary one ; it has also its nucleus. The only circumstance 

 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 

 onlv indication of the nucleus in both instances, d and e 

 elongate at both ends into fibres, one of which (the upper end 

 of d) terminates in a knob with a 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, /is a cell which elongates 

 into fibres on three sides. AVhen a small piece of the 

 skin of the tadpole is torn in water, separate portions of 

 these pigment-fibres, or prolongations of the colls 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 j 

 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 be- 

 come 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 arc in 



