32 I The Process of Evolution 



generally observed behavior of the chromosomes as seen with a 

 light microscope. ( Variations or exceptions have been noted in some 

 organisms or with special techniques.) When a cell becomes a 

 meiocyte, it usually enlarges somewhat, and its nucleus stains more 

 faintly than before. When the chromosomes become visible in pro- 

 phase, they often can be seen to be single strands instead of being 

 double-stranded, as in mitosis. Their subsequent behavior is so 

 complicated that the first meiotic prophase is prolonged in time and 

 has been divided into a series of substages, the names of which need 

 not concern us here. The first occurrence is synapsis of the chromo- 

 somes ( present, of course, in pairs ) with their homologues, precisely 

 point for point along their length. After pairs or bivalents have been 

 formed, the chromosomes then appear double-stranded. (It will be 

 remembered that, in mitosis, prophase begins with the chromosomes 

 double-stranded.) Each bivalent thus comprises four chromatids, 

 two of the chromosome that arrived in the maternal gamete and 

 two paternal chromatids. Any chromosome that does not have a 

 homologue, a sex chromosome for example, remains as a univalent 

 but undergoes doubling at about the same time as the others. 



Apparently at about the time the chromosomes double, the slender 

 chromatids break and rejoin in the bivalents. Intimately associated, 

 coiled, and twisted, they often reunite in nonsister combinations; 

 that is, instead of sister chromatids rejoining, maternal and paternal 

 chromatids may be connected following a break. This is the phe- 

 nomenon of cytological crossing-over. In some organisms crossing- 

 over does not occur in one sex, e.g., male Dwsophilo and Callimantis. 

 When the chromosomes have become double, they behave as if they 

 now repel one another. Bivalents become widely spaced in the 

 nucleus, and members of bivalents are held together only where 

 crossing-over has occurred. (If crossing-over has not occurred, the 

 chromosomes in a bivalent frequently separate at this stage. ) 

 As a result of the repulsion ( this term is used only descriptively ) of 

 the chromosome arms, the bivalent assumes forms that depend upon 

 the number and position of crossovers; the latter now become visible 

 as chiasmata or cross-shaped configurations. 



At this stage of meiosis a chiasma indicates a crossover. Subse- 

 quently, as the chromosomes coil and shorten and become more 

 stainable, the chiasmata (but not the points of crossing-over) are 

 pushed to the ends of the chromosomes. This process, known as 

 terminalization (Fig. 2.5), also produces characteristic configura- 

 tions of bivalents, adjoining loops lying at right angles. 



At the end of the first meiotic prophase, the nucleus contains the 

 gametic number of bivalents ( plus any univalents that are present ) . 

 At the first metaphase a spindle is formed, presumably precisely as 



