34 I The Process of Evolution 



in mitosis, and the bivalents become arranged on its equator. Dur- 

 ing first anaphase, instead of the centromeres dividing as in mitosis, 

 the two centromeres of a bivalent move to opposite poles. Thus the 

 chromosomes do not divide (for a chromosome is defined by its 

 centromere) but disjoin, and disjunction results in two daughter 

 nuclei that undergo the usual telophase transformation (or the 

 latter may be much abbreviated). The centromeres of univalents 

 similarly do not divide, a univalent going to one pole or the other. 

 The distribution of maternal and paternal chromosomes is com- 

 pletely at random. 



During the second division of meiosis, the behavior of the chro- 

 mosomes is like that in mitosis, the difference being that crossing- 

 over has taken place so that the chromatids attached to a centromere 

 are not identical. This second division, in which the centromeres 

 divide, results in the formation of the four daughters of the meiocyte. 

 Each has the gametic chromosome number, but the chances of one 

 daughter being genetically like any other are extremely small. Ma- 

 ternal and paternal chromosomes have been segregated at random, 

 chromatids have been segregated at random, and finally, as the 

 result of crossing-over, the genetic material in the parental genomes 

 has been partially exchanged. 



Fig. 2.5 I Terminalization of chiasmata. From left to right, chiasmata 

 move to ends of ehromosomes. ( Note that the point of crossing-over 

 does not change. ) Far left, cross section of bivalent. Far right, rotation 

 of chromosomes has occurred. {Adapted from De Robertis, Nowinski, 

 and Saez, 1954, General Cytology, 2d cd., W. B. Saunders Company.) 



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