134 MEWS IS IN DIPLOIDS AND POLYPLOIDS 



Consider first mitosis. The chromosomes then consist throughout 

 prophase of pairs of chromatids which appear to attract one another. 

 Each chromosome has a centromere which is effectively single at 

 metaphase and effectively double at anaphase. Its division seems 

 to determine the separation of daughter chromatids. 



At meiosis, on the other hand, the chromosomes are single at the 

 earliest prophase. They unite in pairs, however many potential 

 partners are present. This union ceases when they divide. Asso- 

 ciation afterwards is between pairs of chromatids, and chromosomes 

 repel one another. They are held together merely in so far as their 

 chromatids exchange partners at chiasmata. Moreover, if they 

 happen to divide before pairing is complete, the process is inter- 

 rupted. There is a time limit to pairing. 



There is therefore potential association at all stages of meiosis 

 of pairs of threads — chromosomes or chromatids. The same is 

 true of mitosis. The difference is that the prophase begins earlier 

 in meiosis. The chromosomes are still undivided. Homologous 

 threads attract one another and pairing is possible for a short time 

 before division takes place. The special properties of meiosis 

 follow from the precocity of the prophase and not from the action 

 of new forces. The external and internal processes of division 

 are, as we saw earlier, out of step. 



From this initial upset arise the later special properties of meiosis, 

 the pairing of the chromosomes, their crossing-over, their specially 

 strong contraction at metaphase, the postponement of the splitting 

 of the centromere to the second division ; and possibly the rapid 

 sequence of the two divisions. A simple quantitative time-difference 

 gives rise to the complex series of qualitative differences that 

 distinguish meiosis from mitosis and have determined the origin of 

 sexual reproduction. 



