Meiosis 59 



First Prometaphase. When the nuclear membrane disappears, 

 a spindle forms and the bivalents move towards the equator. 

 The chromosomes are even shorter and thicker than they were 

 during prophase. The chromatids are not usually visible under 

 ordinary methods of staining but can be made to appear by 

 special techniques. 



First Metaphase. AVhen the bivalents reach the equator they 

 arrange themselves on it. They are especially clear in plants 

 with few chromosomes (Figs. 19 and 20). If the plant has both 

 large and small chromosomes, the small ones are usually towards 

 the center (Fig. 21). There are several outstanding differences 

 between metaphase of the first meiotic division and a meta- 

 phase of a somatic mitosis. In a somatic division, the metaphase 

 chromosomes are placed so that their centromeres lie on the 

 equator. In the first meiotic metaphase, the centromeres could 

 not lie on the equator unless the bivalent lay on its side. The 

 bivalents are oriented so that the centromeres are towards the 

 poles and the chiasmata in the equatorial plane. 



The way any bivalent is oriented is purely a matter of chance. 

 That is, the centromere of the chromosome which came from the 

 male parent may point to either pole, and obviously the centro- 

 mere from the original female parent will point in the direction 

 of the opposite pole. Not only is any one pair of chromosomes 

 oriented at random with respect to the poles but each bivalent 

 is also arranged entirely independently of any other. 



First Anaphase. When the bivalents have become arranged 

 on the spindle, they begin to pull apart. This separation is 

 apparently the result of a strong repulsion between the opposite 

 centromeres, wiiich drag the rest of the chromatids after them. 

 When the chiasmata are terminalized, the chromosomes break 

 apart easily as the centromeres move towards the pole; when 

 nonterminalized, the chiasmata slip along towards the ends until 

 the chromosomes have pulled apart. The two sister chromatids 

 are still in contact at the centromere, but the double nature of 

 each chromosome, often completely obscured at metaphase, is 

 now very evident. The anaphase chromosomes are much shorter 

 and thicker than the anaphase chromosomes in a somatic mitosis 

 of the same plant and would hardly be recognized as belonging 

 to the same organism. 



