SECTION g: genetics and cytology 



This means, apparently, that the pairing of chromosomes 

 does not follow from their identity, that is, from any present 

 attraction between them. Rather it can be seen that, con- 

 trary to the usual belief, the only attraction that operates is 

 that between pairs of half-chromosomes. And this attraction 

 is the same as that which is characteristic of mitosis. That 

 pairs of whole chromosomes associate depends on exchanges 

 of partner, chiasmata, amongst the half-chromosomes. 



An earlier stage than metaphase must be sought therefore, 

 the stage at which the behaviour of the chromosomes de- 

 termines the formation of chiasmata, for the essential distinc- 

 tion between meiosis and normal mitosis. This distinction is 

 found in the fact that the chromosome threads are single at 

 the early prophase of meiosis, double at the corresponding 

 stage of condensation in mitosis. 



Can it be, it may be asked, that the chromosomes in pairing 

 at zygotene rectify this discrepancy between meiotic and 

 normal conditions and that their later splitting leads to the 

 falling apart of pairs of half-chromosomes at diplotene? If 

 this is the case, the difference between meiosis and mitosis is 

 able to be described in simple terms, viz. that the prophase 

 condensation or contraction takes place in meiosis before the 

 chromosomes have divided, and not after, as in mitosis. It is 

 then intelligible that the chromosomes should condense further 

 at meiosis than at mitosis, and that cases of the failure of 

 chromosome pairing and suppression of meiosis may be 

 ascribed to specific conditions with correlated effects. Whether 

 or not the meeting of these loops at chiasmata is conditioned 

 by "genetic crossing-over" is an independent problem. 



