MEIOSIS 



germ cells by the parent zygotes there must therefore have been 

 a process of reduction, a halving of the chromosome number to 

 compensate for the addition in fertilization. Such a process, predicted 

 by Weismann in 1887, is now knov^m or assumed to occur in the 

 cell history w^herever there is sexual reproduction. It is called meiosis. 



In most algae, fungi and protozoa the diploid or double nucleus 

 produced by fertilization undergoes meiosis at once to give plants 

 or animals with haploid nuclei containing the half number of chromo- 

 somes. Elsewhere meiosis is postponed so that a diploid stage is 

 intercalated in the life history. In the mosses and liverworts the small 

 diploid organism or sporophyte is, as it were, parasitic on the larger 

 haploid. In the ferns the diploid has become more important than 

 the haploid. In the flowering plants the haploid phases, the embryo- 

 sac and poUen grain, are reduced to a parasitic state. Finally, the 

 complete reversal, in which the haploid phase is eliminated as a 

 separate organism and exists only as the germ cells themselves, eggs 

 and sperm, is attained in all the higher animals. 



Through all these variations in relation to the life cycle the course 

 and character of meiosis remain the same. It may be concerned 

 with the formation of a fungus spore or a lily pollen grain, the 

 maturation of a spider's egg or a human sperm; but it consists always 

 of two divisions of the nucleus, with only one division of the 

 chromosomes. From its very outset it differs from mitosis. The 

 chromosomes are single when they emerge from the resting stage 

 in the mother cell. Moreover, just as in polytene nuclei, where 

 activity and reproduction are going on at once, the chromosomes 

 appear in the form of strings of chromomeres. As soon as they 

 appear, these strings begin to .pair. They come together side by side, 

 chromomere by chromomere. The chromosomes make contact and 

 begin to pair at their ends, or at their centromeres, or at both at the 

 same time. From the contact point the pairing runs along them 

 zip-fashion. Sometimes it runs the whole length. Sometimes it is 

 interrupted so that only the part near the first point of contact is 

 paired: pairing is then locaHzed. After pairing, the chromosomes 

 coil round one another, and so they remain for an indefinite period. 

 This is the thick-thread or pachytene stage. 



We then see that the chromosomes of each pair exactly correspond. 

 Evidently one is from the sperm, the other from the egg, which 



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