YELLOW MICE 209 
The facts are explicable in one of two ways. On the 
earlier hypothesis there was supposed to be some 
obstacle in the way of the fertilization of a yellow- 
bearing egg by a spermatozoon bearing the same 
character. In this case, since spermatozoa are in 
great numerical excess as compared with eggs, it is 
still possible for every Y-bearing egg to be fertilized 
by a spermatozoon lacking Y, as well as half the non- 
yellow eggs by Y-bearing sperms, giving a ratio of 
3:1 in F2 from yellow by yellow. On the other 
hand, it is possible that pure yellow zygotes are formed 
in fertilization, but for some unknown cause are in- 
capable of development. In this case a third of the 
yellow progeny would be wanting, and the expected 
ratio would be 2:1. Cuénot’s figures led to the 
belief that the first hypothesis was the correct one, 
but in further experiments by Castle and by Miss 
Durham the ratio was found to approach 2:1 ina 
majority of cases. 
We have still to describe a case in which two latent 
factors, one derived from each parent, give rise, by 
their simultaneous presence in the zygote produced, 
to the appearance of an entirely new character. The 
following example is the first one of the kind to be 
completely elucidated, and is one of those studied by 
Messrs. Bateson and Punnett and Miss Saunders. 
The white-flowered variety of sweet-pea known as 
Emily Henderson was found to exist in two forms, 
only to be distinguished from one another by the 
shape of the pollen grains which they produced. In 
one of the two the shape of the pollen is elliptical 
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