16 THE METHODS AND 



back of the tinctures again into a double row 

 of bottles, a pair corresponding to each in- 

 gredient ; and each of the germ-cells as then 

 made of a drop from one or other bottle 

 of each pair: and in our model we may repre- 

 sent the phenomenon of segregation in a 

 crude way by supposing that the bottles 

 having no tincture in them, instead of being 

 empty contained an inoperative fluid, say 

 water, with which the tincture would not mix. 

 When the new germ- cells are formed, the two 

 fluids instead of diluting each other simply 

 separate again. It is this fact which entitles 

 us to speak of the purity of germ-cells. They 

 are pure in the possession of an ingredient, 

 or in not possessing it ; and the ingredients, 

 or factors, as we generally call them, are units 

 because they are so treated in the process of 

 formation of the new gametes and because 

 they come out of the process of segregation 



