HEREDITY AND VARIATION 



The essential features of meiosis arc ilh. I in 1 



the case of mitosis it is convenient to distinguish a nu: 

 very complicated proo 



Heterotype I m : Proj > a 



In the first stage {leptotene) (Fig. 430, a, 431, 1) the chroma 

 as long slender threads with a general resemblance t<> the chn 

 in the first stage of mitosis, except that the threads are single instead 

 double. 



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Fig. 431. 



Meiosis— early stages in Trillium erection. 1, leptotene ; 2, EygOteoe ; r ra 

 tene. (From Sansome, after Huskins and Smith, by courtesy of Scientific Horti- 

 culture.) 



At the second stage {zygotene) (Fig. 430, b, 431, 2) the chromosomes become 

 associated in pairs. This association is not a random one ; the chromosomes 

 that pair are the corresponding, similarly constituted or homologous ones, one 

 of each pair being derived from the male gamete, and the other from the female 

 gamete, which initiate this sporophyte generation. 



In the third stage {pachytene, Fig. 431, 3) the paired chromosomes api 

 shorter and thicker as a result of twisting round one another. 



At the fourth stage {diplotene, Fig. 430, c, d) the chromosomes split length- 

 wise into halves so that each pair now consists of four intertwined threads 

 (chromatids), held together in pairs by the centromeres, which do not split. 

 At this stage, or immediately prior thereto, the members of a homologous 

 pair interchange some of their parts, a process known as crossing-over, only two 

 out of the four chromatids breaking and rejoining at any one point. The 

 homologous chromosomes then begin to separate, but not completely, being 

 still held together at certain points— chiasmata— as a result of crossing-over 

 (Fig. 433). These four stages collectively constitute pt ; \ 



Metal 

 During the fifth and sixth stage (diakinesis and metaphass) the four chroma 

 tids continue to shorten and thicken ; the nucleoli disapp ■ Duel 



spindle is formed ; and the chromosome-pairs arrange themselves In the 



equatorial plane. 



At the seventh stage (first anaphase, Fig. 432, 6) the paired chromosomes 

 part company, two chromatids— held together by the unsplit centromere- 

 passing to one pole of the cell and two to the other pole (Fig. 430, /). 



