INHERITANCE OF EYE-COLOUR IN GAMMARUS. 19 
the fact, however (l.c., pp. 543 and 552), no special attention was given to 
the matter. 
The descendants of two original pairs taken in June, 1912, were kept 
under observation for moults, etc., in the Laboratory until August, 1913, 
when the present writers undertook to investigate the variation in 
eye-colour, with a view to determining first if it were a sex-limited 
character, and secondly, if it conformed to the Mendelian law of 
inheritance. 
In the course of these investigations we have received constant assist- 
ance and advice from the Director, Dr. E. J. Allen, F.R.s., at whose sug- 
gestion they were first undertaken, and we wish here to acknowledge 
our great indebtedness to him. 
GENERAL CONDITIONS. 
Before entering into the detailed results of the experiments it will be 
necessary to give a brief description of the habits of the species as well as 
of the conditions under which the animals were kept in the Laboratory. 
Gammarus chevreuxi is an ideal species for experimental work. It 1s very 
hardy, quickly reaches maturity, and breeds all the year round. The 
young are extruded from the marsupial pouch and another batch of eggs 
laid generally within 24 hours of the time of hatching. During the 
summer season a brood takes from 12 to 14 days to hatch, and the period 
of sexual activity is reached at the age of 36 days; in winter in natural 
conditions a brood takes 30 days to hatch, and does not become sexually 
mature for at least 3 months, low temperature, as would be expected, 
retarding development. In the Laboratory, however, which is heated 
in the winter, there is practically no difference in the seasons, and it 
therefore becomes possible to obtain several generations in the year. 
It may be well to state here that in this species of Gammarus the female 
never lays eggs unless a male is present, and also that it is absolutely im- 
possible for a male to fertilise two broods of eggs with one deposition of 
sperm. The male generally takes the female a few days before the eggs 
hatch, and carries it until the young are extruded from the pouch. The 
female then moults, assisted by the male, as described in the paper re- 
ferred to above (J.c., p. 550). The aperture of the oviduct is opened by 
the removal of the old cuticle, and the male deposits the sperm in the 
pouch around it, but unless the eggs are laid within a few hours they 
cannot be laid at all. The cuticle hardens rapidly, and a plug of the 
glutinous lubricatory matter which accompanies moulting and oviposition 
closes the aperture and hardens to the consistency of the cuticle, effectually 
