152 MENDELISM , chap. 



early in the uterus. Quite recently additional con- 

 iirmation has been forthcoming from the work of 

 several American investigators. They have found 

 that when the uterus is opened during the earlier 

 part of pregnancy, dead embryos are much more 

 numerous in yellow females that have been mated 

 with yellow males, than in yellow females bred to 

 non - yellow males. The unexpected complication 

 arising from the failure to realise the looked - for 

 3 : I ratio, when heterozygous yellows were bred 

 together, led to the interesting and important dis- 

 covery that certain gametic combinations may lead 

 to the formation of a zygote which, for no apparent 

 reason, is doomed to perish prematurely. Since the 

 discovery of this phenomenon in the mouse, evidence 

 has come to light that it occurs also in Drosophila, 

 and it is not unlikely that it may prove to be wide- 

 spread in both animals and plants. 



For another interesting complication we may 

 turn to plants. In the common English bryony 

 (Bryonia dioica) the sexes are separate, some plants 

 having only male, and others only female flowers. 

 In another European species, B. alba, both male and 

 female flowers occur on the same plant. Correns 

 crossed these two species reciprocally, and also 

 fertilised B. dioica by its own male with the follow- 

 ing results : 



dioica ? x dioica $ gave ? ? and $ S 



„ X alba $ „ 9 ? only 



alba 9 X dioica $ „ $ $ and $ i . 



The point of chief interest lies in the striking differ- 

 ence shown by the reciprocal crosses between dioica 



