SIGNIFICANCE OF ACTION OF DIPLOIDS 263 



in some way or change individually in some way so that they can 

 give rise to one or more (ab) nuclei which can pass into the (AB) 

 mycehum and diploidise it in the normal manner ? 



If it should be found that, in an illegitimate combination of a 

 haploid mycehum with a diploid mycehum, the diploidisation of 

 the haploid mycelium is due simply to " Durchbrechungskopula- 

 tionen," another question suggests itself : how is it that, in illegiti- 

 mate combinations, a large haploid mycelium can be diploidised by 

 a diploid inoculum but not by either of the components of the 

 diploid inoculum ? To cite a concrete example of this question : 

 how is it that a large mycehum (AB) can be diploidised by a diploid 

 inoculum {Ab)-^{aB), but not by a haploid inoculum (^6) or by a 

 haploid inoculum (aB) ? 



In the four experiments recorded in Table IX, as we have seen, 

 the diploid inoculum diploidised the large haploid mycelium and 

 then the diploidised large haploid mycehum diploidised both of the 

 isolated haploid inocula. Progressive diploidisation of this kind 

 doubtless takes place under natural conditions in horse-dung balls ; 

 for, in these substrata, haploid and diploid mycelia of Coprinus 

 lagopus must often meet one another and form all sorts of illegiti- 

 mate combinations. The study of illegitimate combinations be- 

 tween haploid and diploid mycelia is therefore not merely of 

 genetic interest, but is of importance for elucidating the hfe-history 

 of Coprinus lagopus and other Hymenomycetes. 



The Biological Significance of the Diploidisation of a Haploid 

 Mycelium by a Diploid Mycelium. — In previous pages of this 

 chapter it has been demonstrated that, in Coprinus lagopus, a 

 typical Hymenomycete, a diploid mycelium {AB)'^{ab) can readily 

 diploidise the haploid mycelia (AB) and (ab), and that a diploid 

 mycelium {Ab)-\-{aB) can readily diploidise the haploid mycelia 

 (^6) and (aB) ; and evidence was adduced to show that, in these 

 so-called legitimate combinations, the haploid mycelium becomes 

 diploidised by nuclei of opposite sex derived from the diploid 

 mycelium. Thus, in the combination (-4^) X {AB)-\-{ab), diploidi- 

 sation of the haploid mycehum (AB) is effected by one or more 

 (ab) nuclei, derived from the conjugate nuclei of the diploid myce- 

 hum {AB)~\-{ab). Furthermore, it was shown that in the so-called 



