REDUCTION AND RECOMBINATION 



daughter nuclei at once divide again. But their chromosomes, already 

 double, do not divide again. They lie on the second metaphase plate 

 and look just like those of an ordinary mitosis, except that their 

 chromatids, which are not true sister chromatids throughout their 

 length, do not lie together throughout their length. Indeed they 

 need not touch at all except at their centromeres. The second meiotic 

 anaphase then enables us to see very clearly the essential character 

 of an anaphase. Separation must depend on the division of the cen- 

 tromeres, because elsewhere the chromatids are already separated. 

 The result is four nuclei, each with the haploid number of chromo- 

 somes, now normal single chromosomes, in fact four truly haploid 

 nuclei (Fig. 6). 



Reduction and Recombination 



What does meiosis tell us ? The chromosomes which pair, indeed 

 the individual chromomeres which pair, are generally of similar size 

 and shape. They must therefore pair because they are like one 

 another. And since the differences in the sizes and shapes of different 

 pairs are constant from generation to generation, the pairing chromo- 

 somes must be derived from opposite parents in which they have 

 corresponding structures and functions. They must therefore (ulti- 

 mately) be of common ancestry, or, as we say, homologous. Further- 

 more the chromosomes which do not pair, whether from the same 

 parent or from opposite parents, must be different in structure and 

 function from one another. To some extent, the chromomeres which 

 make up each chromosome may be supposed to be carried along by 

 their neighbours in pairing. But, to some extent, they too must be 

 differentiated particles with their own specific attractions for similar 

 homologues. In other words, the chromosomes composing a haploid 

 set must be different from one another and are likely to be made 

 up of specific and different chromomeres in a constant linear 

 arrangement. 



What are the results of meiosis ? The diploid mother-cell has given 

 rise to four haploid spores or gametes. The number of the chromo- 

 somes has been reduced to half. There was formerly a great deal 

 of argument as to whether this reduction took place at the first 

 division or at the second. We now see that it takes place at neither. 

 It takes place by virtue of the fact that no division of the chromo- 



Elements of Genetics "IT. C 



