348 Eye-Colour in Gammarus 



over the surface of the whole ommateum, the lenses showing as circles 

 of colour enclosed in a white meshwork or reticulation. 



The white pigment disappears within an hour or two of death, and 

 is soluble in all the usual preserving fluids. 



The black retinal pigment on the other hand is practically permanent; 

 the red lasts for some time in preservatives but eventually fades out. 



Mated together, Black proved dominant to Red, giving ^2 in the 

 proportion 3 : 1, and three stocks were established — Pure Black, Hybrid 

 Black (i.e. Black carrying Red), and Pure Red^ 



Colour is always associated with the Perfect Form. In only one case 

 has an exception been noted. This was in the Pure Red stock where 

 two succeeding broods of Reds each contained one specimen in which 

 the form of the eye was perfect, but the retinal cells were unpigmented. 

 One, a male, survived to mate, and was put with a degenerate-eyed 

 Albino female (see Imperfect Form). The result of this mating was 

 a brood of coloured offspring. Experiments (given in detail in Allen and 

 Sexton, 1917, pp. 287-325) showed the lack of colour to be due to some 

 obscure pathological cause, the animal functioning as a Normal Red-eye. 



The same thing, but in a much less degree, has occurred in several of 

 our stocks, animals both Black and Red were found with patches of 

 uncoloured ommatidia in otherwise normal eyes. These were bred 

 together but in all cases the succeeding generations were normal-eyed, 

 proving the condition to be not inheritable. 



The stock, from which the two white-eyed specimens described above 

 were derived, was kept under observation until it died out, November, 

 1918, but beyond a few instances of dilute Red pigment, nothing unusual 

 has been noted. 



No second case of the Red-eye mutation has arisen independently 

 up to the present date. 



Darkening of Red pigment with age. It must be noted here that 

 the retinal pigment in Red eyes is not always of the normal bright- 

 red tint. It tends to darken with age; occasionally all the ommatidia 

 become so dark as to appear black, in other instances dark ommatidia 

 develop among the red, especially in the middle of the eye. In the case 

 figured, that of the female I.E. (see Plate XIV, Fig. 2, and p. 356) which 

 had the record number of young, 780 in all, the dark pigment increased 

 with age, until the eye looked black. The figure was drawn not long 



1 In this paper we use the term "Pure" in reference to colour simply to denote 

 the absence of the Albino factor. Pure Black, Pure Red, means Black or Eed without 

 the Albino factor, " Hybrid " Black means Black carrying Red, but no Albino. 



