the Sense of Sicjht. 163 



the wandering-cells into those of other tissues to which they 

 have been brought. Just as the cells of the tissue where 

 they originated passed theui on to the wandering-cells, these 

 can again pass them on to other cells. I may, perhaps, 

 recall the collections of pigment-cells within the ovaries of 

 frogs, which appear to be passing their coloured granules into 

 the eggs. 



Chief among the tissues receiving the loads brought to 

 them by the wandering-cells is the outer skin. Here they 

 can be seen discharging their granules to the epidermal 

 cells, which utilize them in various ways. They take part — 

 probably a very important part — in the formation of epidermal 

 protective structures, chitinous and horny cuticles, &c. Slime- 

 glands appear to use them in the formation of their secretions. 

 Or, again, they are stored up, altered or unaltered, as the 

 colouring-matters of the skin and of its derivatives. 



Remarkable among skins coloured by these granules are 

 those endowed with what is known as the chromatic function. 

 In these cases the wandering-cells have either been arrested 

 in the connective tissue just beneath the skin, or have given 

 up their granules to connective-tissue cells, the resultant 

 " chromatophores" coming under partial control of the nervous 

 system. The highest specialization of this is seen when, in 

 two or more layers of cells containing differently coloured 

 granules, these granules change their positions in the bodies of 

 the cells, spreading out or crowding together at different times 

 and in different degrees, so as to produce an astonishing 

 play of colour. But even in such cases it is probable that 

 individual granules escape from these secondary detentions and 

 reach the outer cells of the skin, where they may be needed 

 for various more primary functions. 



Leaving out of account those cases which are complicated 

 by secondary association with the nervous system, we have 

 abundant evidence to show that, although the wandering-cells 

 under discussion somehow travel to their respective destina- 

 tions in the dark, they are very sensitive to light, towards 

 the source of which they move. This is a common pheno- 

 menon among unicellular organisms both animal and vege- 

 table, and it is specially marked when the contents of the 

 cells are coloured. It is in large measure to this attraction 

 that the great abundance of tiie granule-bearing cells or of 

 transported granules in or under the skin is due. 



The theory which 1 desire to propound is, that it is to this 

 constant striving of wandering-cells to travel towards the light, 

 and, if possible, to escape altogether from the cell-complex 

 of the body in which they originated (or, perhaps, only to 



11* 



