LOSS OF EYE-PIGMENT IN GAMMARUS. 219 
ommatidia arranged in regular rows, each ommatidium being surrounded 
by pigmented retinular cells, the pigment being black in the normal 
eye, red in the mutation. On the surface and around the upper portion 
of the ommatidia, a chalky white extra-retinal pigment is found, the 
“accessory pigment,” which gives the reticulated appearance to the 
ommateum. In the albino eye only this extra-retinal pigment is 
developed. (Plate VII. Fig. 4.) The ommateum is much altered, 
is reduced considerably in size, and is very variable in shape, even 
the eyes of the same animal often differing widely in form, and in the 
size, shape, number and arrangement of the ommatidia. The surface of 
the eye is flat, not convex as in the type, with a few ommatidia sparsely 
scattered, generally around the margin, and with some occasionally 
lying beyond it. Especially portions of the extra-retinal chalk-white 
pigment tend to become detached, causing white spots to appear in more 
or less definite positions on the head. A more detailed study of these 
spots is still in progress. 
The Albino eye appeared in the F, generation from a mating of Pure 
Biack with Pure Red. The young (F,) of this mating all had normal 
black eyes. The 15 which survived to maturity were kept together in one 
bowl to breed, each female when ovigerous being removed to a separate 
bowl until her young were hatched, and then returned to the brood- 
bowl to mate again. The forty-second brood (F,) obtained from this 
family consisted of 7 Black-eyed young, 1 Red-eyed and 4 with neither 
black nor red pigment, the Albino eye just described. The total number 
of young recorded from all the broods was 745, of which 559 were black- 
eyed, 182 red-eyed and 4 albino-eyed. The four albinos reached maturity, 
one male and three females, but only two females survived to produce 
offspring, the AB and AC (Plate IL) of the following experiments. 
All the albino-eyed animals used in the experiments are descendants 
of these, and there has been no other case observed of an independent 
origin of this mutation. The stock from which these two females came 
was kept for a further period of eighteen months, and no more albino 
eyes occurred in it. 
Cross A. 
CROSS BETWEEN THE ALBINO FEMALE AC AND A PURE 
Rep MALE R.2. (Plate I.) 
One of these females (AC) was mated with a male from Pure Red 
stock (R.2), the resulting offspring being 3 black and 6 red-eyed young. 
This at once suggests that colour is dominant to absence of colour, and 
that the albino eye, in which only white accessory pigment appeared , 
contained the factors for both black and red retinal pigment. 
