386 GENERAL PRINCIPLES 



results are as follows: We will take as an example the pure 

 white and common gray mice. The result of this cross is 

 gray mice like the gray parent, not a lighter gray, as one might 

 expect, following the case of Mirabilis Jalapa. In the second 

 generation there are a fourth white, which are pure, and three- 

 fourths are gray. The gray are in reahty of two kinds, though 

 this becomes evident only in the course of succeeding genera- 

 tions, when it develops that one-third of the three-fourths 

 continue to breed only gray, while the other two-thirds yield 

 one-fourth white in the succeeding generation and are thus 

 seen to have been mixed. The second generation of offspring 

 may, therefore, be described as one-fourth white, two-fourths 

 mixed with gray dominant, and one-fourth pure gray. The 

 dominant grays and pure grays can only be distinguished by 

 the character of their offspring. 



784. Not every character is controlled in this simple way, 

 for it is readily conceivable that a given character may be due 

 to the combined operation of several factors, each of which may 

 be separately heretible. 



785. Physical Basis of Heredity. — The phenomena of he- 

 redity correspond in a remarkable way with those of matura- 

 tion, which makes plausible the theory that the chromosomes 

 are the bearers of the hereditary traits of the organism, and 

 that during maturation the readjustment of chromatin deter- 

 mines the ancestral characters which are to be handed on to the 

 next generation. In the process of fertilization the germ cell 

 is provided with equal parts of maternal and paternal chroma- 

 tin, and this condition is maintained during the subsequent 

 stages of development in all the cells of the body. Let the 

 condition of the chromatin with regard to any hybrid character 

 be represented by (P.M.). Then when the first maturation 

 division occurs in the primary oocyte of the hybrid (F.) the 



(P.M.) elements divide into 



(P.M.) = ist polar body 

 (P.M.) = secondary oocyte. 



