MEIOSIS 



appearance of the nucleolus and the nuclear membrane, the appear- 

 ance of the spindle, and the movement of the bivalents on to the 

 metaphase plate, in other words, by metaphase. 



But this, the first metaphase of meiosis, is different from metaphase 

 of an ordinary mitosis in two related and all-important respects. 

 First, individual chromosomes are not now sufficient to themselves. 

 Their centromeres do not orientate themselves separately or individu- 

 ally on the spindle. They orientate themselves only in relation to their 

 partner centromeres. The stretching between partner centromeres, 

 already noticeable at diakinesis, is now exaggerated. Secondly, and 

 consequently, anaphase does not begin by the division of the cen- 

 tromeres. Just as the chromosomes remained undivided at the be- 

 ginning of prophase, so, in turn, the centromeres remain undivided 

 at the beginning of anaphase. It is not their division which sets the 

 chromosomes moving to the poles. It is merely the lapse of attraction 

 between the paired chromatids where they are hindering the move- 

 ment apart of the paired chromosomes, i.e. on the far sides of the 

 chiasmata from the centromeres. Such a lapse of attraction occurs 

 at mitosis, but it is hardly noticeable because at mitosis there is 

 nothing to puU the chromatids apart until the centromeres divide. 

 Now the paired centromeres are already repelling one another in 

 readiness for the lapse of attraction. When it occurs, the daughter 

 bivalents, each double except for its centromere, run to opposite 

 poles. The first division is complete. 



The situation as the daughter nuclei are formed is a remarkable 

 one. Each nucleus has the haploid number of chromosomes, but the 

 diploid number of chromatids. For the chromosomes are already 

 divided. They are double as they are in the prophase, not in the 

 telophase, of an ordinary mitosis. 



Nor are the two chromatids of each chromosome related to one 

 another like those in prophase of mitosis. They are true sister chroma- 

 tids between the centromere and the nearest chiasma. Beyond that 

 chiasma, until the next one, they are chromatids from parmer 

 chromosomes. Looking at it from another angle, we may say that 

 the separation of the first division has been between partners next 

 to the centromere: it has been reductional. Beyond the chiasma it 

 has been between sister chromatids : it has been equational. 



An unexampled situation calls for an unexampled remedy. The 



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