64 Reproduction and Meiosis 



often lie close together as in a somatic mitosis, although they 

 sometimes diverge. Where there is no interphase, the spindle 

 of the first division seems to break up into two spindles just 

 as the anaphase chromosomes of the first division have reached 

 the poles, and the two groups of anaphase chromosomes move 

 immediately on to the equatorial plates of the new spindles and 

 become the second metaphase chromosomes. They usually elon- 

 gate in the process. 



Second Anaphase. Second anaphase begins when the daughter 

 centromeres pull apart towards the opposite poles. The chromo- 

 somes of second anaphase are not the short, thick bodies of the 

 first meiotic division but are much more like the anaphase 

 chromosomes of a somatic mitosis. 



Second Telophase. When the anaphase chromosomes reach 

 the poles, new nuclei form in the usual manner. The chromosomes 

 lengthen and almost completely uncoil, and nuclear membranes 

 and nucleoli appear. Cell walls usually divide these two cells 

 into four although occasionally no walls form, as in megasporo- 

 genesis of the lily. In organisms in which cell walls do form, if 

 a wall did not form during the first telophase, the one cell with 

 four nuclei now becomes divided into four cells. - 



Reduction 



If an animal has 16 somatic chromosomes, at leptotene there 

 would be 16 chromosomes and therefore 16 centromeres. After 

 pairing and ''splitting" of the chromosomes there would be 32 

 chromatids; but since the centromeres either do not divide or 

 divide but remain with the daughter centromeres in very intimate 

 contact during the first division, only 16 effective centromeres 

 would still be present. At first anaphase, 8 effective centromeres 

 and therefore 8 chromosomes would pass to each pole. The fact 

 that each chromosome was composed of 2 chromatids would not 

 make it more than one chromosome for, as long as the centro- 

 meres are intact or together, the chromosomes behave as a unit 

 irrespective of the number of chromatids of which they are com- 

 posed. Therefore, at first anaphase, 8 chromosomes go to each 

 pole. In the second division, the daughter centromeres separate 

 so that each chromosome now becomes two separate units. As a 

 result of this separation of the centromeres, 8 chromosomes go 

 to each pole at the second anaphase. 



