238 COLOUR IN NATURE chap. 



black spots were produced by accumulations of black 

 pigment in the cutis of these regions, in other re- 

 spects the skin showed no special peculiarities. He 

 therefore set himself the problem to find out what 

 anatomical or developmental necessities produced 

 these aggregations of pigment. 



The ringed snake is marked by three paired 

 longitudinal rows of black spots, which correspond 

 to the longitudinal lines described by Eimer in the 

 wall-lizard, except that in the lizard there is, in ad- 

 dition, an unpaired median line on the dorsal surface. 

 In the embryo at an early stage the surface is marked 

 by three pairs of red lines, and a slender median 

 unpaired one. The lines are produced by the sub- 

 jacent blood-vessels and are not necessarily perfectly 

 continuous, being sometimes broken into red spots ; 

 they are connected with numerous transverse vessels, 

 the junctions being marked by distinct swellings of 

 bright colour. Of the three lateral lines the median 

 is the most distinct, and extends forward in front of 

 the eye. A little later the scales begin to develop 

 from before backwards, and as they become distinct 

 the red lines disappear before them, and, at the same 

 time the first traces of pigmentation appear as a 

 longitudinal row of pigment spots at each side, 

 developed in the position previously occupied by the 

 median red line already mentioned. The extension 

 of this line in front of the eye is now marked by a 

 distinct spot of pigment, which forms a marked feature 

 in the adult. The formation of this middle row of 

 spots is followed a little later by the appearance of 

 two other rows corresponding to the upper and lower 

 of the lateral red lines. The first spot of the middle 



