MEIOSIS 



109 



noticeably thicker than in the leptotene (thin-thread) stage. Tiie thick 

 double pachytene threads are present in the monoploid number, and 

 each of them is bivalent, since it consists of two homologous chromosomes 

 in synaptic association (Figs. 77-79). They may be arranged at random 

 or in the bouquet position. 



Late in the pachytene stage each of the two chromosomes of each 

 synapsed pair becomes visibly double, presumabl>^ because of the enlarge- 









'^-^ 



WJ 



'%-'. 



Flu. 78.- — Leptotene, zygotene, pachytene, and very early diplotene stages in niicrosporo- 

 cytes of Trillium. Arrows indicate chiasniata. {After C. L. Huskins and S. G. Smith.) 



ment of the structures concerned. If the doubleness reported in the 

 premeiotic telophase has persisted, although invisible, this represents its 

 reappearance. The pachytene threads are now quadruple: each is a 

 tetrad of chromatids. 



Diplotene Stage. — In each of the tetrads the four chromatids begin to 

 separate, one pair of sister chromatids from the other two, as though the 

 synaptic force holding them together were being replaced by a repulsive 

 force. At one or more points they are prevented from separating by 



