Meiosis 



55 



somes are polarized they are sometimes said to be in the bouquet 

 stage. 



Zygotene is followed by pachytene, a stage characterized by 

 several important featm'es. The two homologous chromosomes 

 which had paired in zygotene now twist about one another in 



Fig. 17. Chromosome pairing in which one of the pairing homologues 

 has a deleted or an inverted segment: (a) a terminal deletion; (b) an 

 intercalaiy deletion; (c) an inversion. (Courtesy of Dr. B. McClintock 

 in Research Bulletin of the University of Missouri Agricultural Experiment 

 Station.) 



what are called relational coils (Fig. 18), and each soon becomes 

 a double structure. It is immaterial for our purpose whether 

 this is because two chromatids in very close contact and consti- 

 tuting one chromosome become so separated now as to be visible 

 as a double structure or whether one original thread w^as present 

 which now forms another thread like it and alongside it. It is 

 often stated that at this time "the chromosomes split longi- 

 tudinally," but it is more likely either that two intimate chro- 

 monemata separate or that one thread regenerates another. 

 After this doubling occurs, the two chromatids of one chro- 



