xiv] LINKAGE IN MALE AND FEMALE 235 



chromosomes. The more fundamental reasons for this differ- 

 ence between the sexes is still quite obscure, but indications 

 of the immediate cause of it are given by the observations 

 of C. W. METZ on Drosophila. 



He writes 1 "In the female the critical stages of matura- 

 tion appear to be of the conventional type. That is, long, 

 delicate paired threads are evident early in the growth 

 period. These are polarised [i.e. contracted to one side of the 

 nucleus in synizesis] and apparently conform to the usual 

 diplotene threads 



"In the male the process seems to be very different 

 throughout. The chromosomes apparently remain in a 

 fairly condensed state during the whole growth period, 

 without transforming into delicate polarised threads or 

 loops as they do in the female.... This would indicate that 

 there is no opportunity for crossing over in the male." 



In confirmation of the fact that this difference in the 

 nuclear behaviour of the two sexes in Drosophila is the cause 

 of the difference in respect of crossing-over, it should be 

 mentioned that PLOUGH (1917) finds that by subjecting 

 female pupae to different temperatures the amount of 

 crossing-over can be modified, and his experiments indicate 

 that crossing-over must take place in the early stages of 

 the oocytes those stages, in fact, in which METZ reports 

 differences in nuclear behaviour in the two sexes. 



Finally, a short space must be devoted to the hypotheses 

 or suggestions that have been made with regard to mode of 

 action of the chromosomes in determining sex and in trans- 

 mitting the factors for Mendelian characters. It has been 

 seen that in most Insects and in animals of a number 

 of other groups the male has one Jf-chromosome, either 



1 Quotation from a letter which the writer kindly gives permission to 

 publish. 



