THE PHENOMENA OF MUTATION 183 



Perhaps the most interesting of the researches on 

 the phenomena of mutation are tliose concerning the 

 relation of the characters to the chromosomes of the 

 cell, in which Gates has been a pioneer and one of 

 the most industrious and successful investigators. 

 The behaviour of the chromosomes in meiosis or re- 

 duction division both in the pollen mother-cells and 

 in the megaspore mother-cells which give rise to the 

 so-called embryo-sac are fully described by Gates. 

 Here it is only necessary to refer to the abnormahties 

 in the reduction division which are related to muta- 

 tion, and the results of these abnormalities in the 

 number of chromosomes. The original number of 

 chromosomes in CEnothera is 14. In the mutation 

 lata this has become 15, and also in another mutation 

 called semilata. The chromosomes before the re- 

 duction division are arranged in pau's, each pair 

 consisting, it is believed, of one paternal and one 

 maternal chromosome. One of each pan* goes into 

 one daughter-cell and the other into the other, but 

 not all maternal into one and all paternal into the 

 other. Thus each daughter-cell after the first or 

 heterotypic division in normal cases contains 7 

 chromosomes. A second homotypic division takes 

 place in which each chromosome splits into two as 

 in somatic divisions, and thus we have 4 gametes 

 with 7 chromosomes each. Now when lata is pro- 

 duced it is believed that in the heterotypic division 

 one pair passes into one daughter-cell instead of one 

 chromosome of the pair into each daughter-cell, the 

 other pairs segregating in the usual way. We thus 

 have one daughter-cell with 8 chromosomes and the 

 other with 6. This 6+8 distribution has actually 

 been observed in the pollen mother-cell in rubrinervis. 



