Hairs and Feathers 55 



marked and interesting way the pigment cells on 

 which principally depends the colour of the hair. In 

 the hair of the white mouse (Plate V., fig. 1) these 

 cells cannot be perceived ; but in those of the common 

 mouse (fig. 2) there is a scrap of colouring matter in 

 each cell, and those little dark spots, seen through the 

 transparent surface of the hair, give to it its dark 

 colour. Yet how is it about the position of these 

 dark spots ? for when we compare figs. 1 and 2, we 

 observe that the remarkable projections (reminding 

 one of a succession of vertebrce) are alike empty in both 

 figures, the little blocks of colouring matter occupy- 

 ing the intervening spaces. Are these projections 

 then not cells after all, but solid partitions of some 

 kind ? No, they are air-filled cells ; we know this by 

 their appearance when the hair is mounted in balsam, 

 for they absorb light, and assume a dark and decided 

 appearance, just as the obnoxious air-bubbles which 

 so often annoy the microscopist when mounting 

 objects; and moreover we can often contrive by 

 heating the glass slide over a flame to get some of 

 the air out of the end of the hair, and completely 

 penetrate it with the balsam. Then the white mouse's 

 hair becomes nearly invisible, except about the point 

 where the balsam ceases to penetrate, and there a cell 

 or two remains only half full of air, resolved into a 

 round bubble ; and the brown mouse's hair shows the 

 portions of " pigment" standing out in beautiful 

 distinctness and regularity. It would seem that there 

 are two kinds of cell in the pith of the mouse's hair, 

 which we may call vacant cells and pigment cells, the 



