244 THE THEORY OF THE GENE 



These four groups as Kniep first showed, can be ac- 

 counted for on the assumption of two pairs of Mendelian 

 factors that may be designated as Aa and Bb. If these 

 factors are segregated, at the time when the spores are 

 formed on each basidium, there will be present on each 

 toadstool four kinds of spores, AB, ab, Ab, aB, each of 

 which gives rise to a mycelium of the same genetic com- 

 position. As shown in Table 1, only those mycelia differ- 

 ing in both factors unite and form clamp connections. This 

 means that there are four sexes, and only those with dif- 

 ferent sex factors can combine. 



There is also a cytological background that accords ex- 

 cellently with these genetic assumptions. The nuclei in the 

 monosporous mycelia lie singly in the protoplasm. After 

 the union of two mycelia, the nuclei in the derived (sec- 

 ondary) mycelium are in pairs. It is reasonable to assume 

 that one member of each pair has come from one my- 

 celium, the other one from the other mycelium. At the 

 time when the four spores are about to develop, reduc- 

 tion is supposed to take place, so that each spore comes 

 to contain a reduced nucleus. Each spore gives rise to a 

 new reduced mycelium. Such a picture accords with the 

 reduction process in higher plants and animals and serves 

 to bring these moulds into line with the genetic results 

 that take place w^hen the chromosomes are reduced from 

 diploid to haploid gametes. It is true that, as yet, it has 

 not been shown in Coprinus and related species that we 

 are dealing with a diploid-haploid relation, but it seems 

 not improbable that this is a correct interpretation of the 

 facts as far as known. If so, the sorting out of the genetic 

 factors in these toadstools is the same in principle as that 

 shown by other plants and animals. 



The preceding relations hold for strains occurring in 

 any one locality. If strains from different localities are 

 tested, an extraordinary result is found. All the monos- 



