1 6 Purity of Type [CH. 



qualitative segregation occurring in one or more of these 

 cell-divisions that allelomorphism depends. The opposite 

 members of each pair of characters being allelomorphic to 

 each other, every zygote*, or individual produced in ferti- 

 lisation, must, in respect of any such pair, be either a 

 homozygote, that is to say, a zygote formed by the union of 

 two gametes each bearing the same allelomorph, as AA 

 and aa, or a keterozygote formed by the union of two germs 

 bearing different allelomorphs, as Aa. Therefore in respect 

 of any pair of allelomorphic characters, the individuals 

 composing the whole population are of three kinds only : 



1. Homozygotes of the form A A, 



2. Homozygotes of the form aa, 



3. Heterozygotes of the form Aa. 



The gametes are of two kinds only, A and a. Each 

 kind of homozygote is pure to the character of the gametes 

 which compose it. 



Purity of Type. 



Purity of type thus acquires a precise meaning. It is 

 dependent on gametic segregation, and has nothing to do 

 with a prolonged course of selection, natural or artificial. 



All this is of course consonant with the visible facts 

 that have been discovered by the cytologists, in so far as 

 the nucleus of each somatic cell is a double structure, while 

 the nucleus of each gametic cell is a single structure. It is, 

 in my judgment, impossible as yet to form definite views 

 as to the relations of the various parts of the cell to the 

 function of heredity. The details of cytology and their 

 interpretation are beyond our present province, but this 

 much is certain : that when in these discussions we idealize 

 the characters as borne by the gamete in an unpaired state 

 and by the zygote in a paired state, we make no assumption 

 which is not in full accord with histological appearances. 



From the fact that the development of characters in 

 animals or plants depends on the presence of definite units 



* In botany the term zygote is usually restricted to the single cell 

 which results from the process of fertilisation, but by a natural extension 

 the word may be used for the individual which develops by somatic 

 divisions from that cell. 



