354 Eye-Colour in Gammarus 



median-lateral spots on the head (1 and 4), suggest a segmental arrange- 

 ment. Are we dealing with the loss of factors, which is leading to a 

 more primitive arrangement, giving traces of eyes on two head segments 

 and on the 1st peraeon ? 



It should be mentioned that the spots recorded were seen in the 

 young when they first emerged from the brood pouch. As a rule the 

 spots remain through life, sometimes changing their shape after a moult 

 (as, e.g. a streak may become a spot, or a patch may break up into 

 several streaks), but very rarely altering in position. Sometimes, but 

 not often, they disappear as the animals get older and become mature. 

 In five instances recorded in our normal stock, four animals hatched 

 with lateral spots, and one, hatched with a dorsal patch, all lost them. 

 In other cases, chiefly among the Albino stock, spots develop with 

 age. There are several records of spots changing their position ^ 



In about a dozen out of many thousands of wild Gammarus chevreaxi 

 examined similar spots have been found. 



The spots appeared almost simultaneously in our different stocks 

 about a year after the " No- white " mutation arose. 



1. The Appearance of Spots in the Pure Red Stock. This stock 

 consisted of the F2 Reds from the crosses between Pure Blacks and 

 Pure Reds, separated in January, 1914, and left to interbreed. 



When examined on Aug. 26, 1915, all had eyes of the usual bright 

 red colour, except four very large animals, three males and one female. 

 In these the eyes had a very curious appearance, some of the ommatidia 

 being unpigmented ; others red, of which some had apparently only 

 part of the retinal cells pigmented, giving the effect of bars or streaks 

 of colour across the cones ; while several ommatidia were black or 

 purplish-black (Plate XIV, Fig. 7). 



This composition of the eye we now consider to be due to age 

 (cf. $ I.E., p. 348), but at the time it was not recognised as such and 

 experiments were carried out to see if it proved inheritable. 



The Pure Red stock was examined again on Dec. 6, 1915 ; 106 



^ 1. An Albino male, hatched with large spot, left side, position 1 ; at maturity the 

 spot was connected with the eye by a little neck of white pigment. 



2. An Albino male, hatched with streak, right side, position 1 ; two months later, the 

 spot was connected with the eye, a month later still, the spot had completely coalesced with 

 the eye. 



3. A Bed female, hatched with spot, right side, position 3 ; at maturity the spot was 

 connected to the eye. 



4. An Albino, hatched with many spots ; the positions changed with each moult, the 

 only record we have of such an occurrence. 



I 



