350 Eye-Colour in Gammarus 



Black-eyed young. This pair (the C. 176 referred to on p. 358) had 12 

 broods, numbering 249 young in all. The first 8 broods were as 

 follows :— 15, 30, 22, 7, 22, 37, 30, and 12, all Albinos, and then came 

 a brood of 2 only. The two young were seen moving in the pouch the 

 day before extrusion. When extruded they proved to be one Black 

 perfect-eyed animal and one Albino. The Black one had a spot of 

 white pigment on the right side, on the edge of the brain, the Albino 

 one had eyes of nearly perfect form, but with the ommatidia few in 

 number, and arranged around the margins. Both died before reaching 

 maturity. There was an interval of two months without any young 

 being hatclied, and then broods of 25, 42, and 5, all Albinos. 



As with the Red-eyed mutation, no second case of Albino eyes arising 

 independently has been seen up to the present time. 



III. The No-white Mutation. 



The white pigment is subject to great variation. In the coloured 

 eyes it occasionally appears as a very heavy reticulation almost obscuring 

 the ommatidia, and accompanied by a loss of brilliancy in the coloured 

 pigment : or it may be so much reduced in quantity that the reticulation 

 only shows as a thread-like network between the ommatidia: or, as 

 happens in many cases, only streaks or flecks of white are left. When 

 it is entirely lacking, the typical No-whites are produced. 



These No-whites appear in both the Perfect and Imperfect Forms, 

 the Absence of the white pigment acting as a simple recessive to its 

 Presence (Plate XIV, Fig. 4). 



The Albino No- white we call the " Colourless " eye, as it lacks both 

 the coloured and the white pigment (Plate XIV, Fig. 5). 



The No-white mutation seems by no means an uncommon one. It 

 has appeared in some of the families in all our stocks of Gammarus 

 chevreuxi, and probably occurs in other Amphipod species. Owing to 

 the white pigment disappearing in preservatives, it is not possible to 

 judge from pickled specimens, but one case has been noted in the living 

 ones. Another species of brackish water Gammarus inhabits the ditches 

 with G. chevreuxi, and one animal was found, in a dredging, with both 

 eyes No-white. 



Origin of No-iuhite Mutation. This mutation begins in at least two 

 different ways : {a) it may appear suddenly either with one eye or with 

 both eyes affected ; or (6) it may develop gradually, e.g. parents with 

 " thin reticulation " giving some young with thin reticulation, and some 



