SEMI-PRECOCITY 407 



somes cross over and form chiasmata only at the moment of 

 division . 



On this view failure of metaphase pairing is, like localisation, a 

 result of reduced precocity, but unHke localisation it is a disordered 

 result ; it depends on an incomplete pairing at pachytene, which is 

 variable in its distribution, and gives rise to a reduced chiasma 



I8jo 



JO.z 





T^ 



Fig. 125. — Different degrees of reduced precocity and reduced 

 pairing in different inbred plants of Secale cereale. X 1000. 

 (Lamm, 1936.) 



formation, which is variable in its frequency. This assumption 

 explains the enormous variation in chiasmata from nucleus to 

 nucleus as the result of variable precocity and the enormous varia- 

 tion from bivalent to bivalent in the same nucleus as the result of 

 chance variation in position of the homologous threads in the 

 zygotene nucleus. We might then expect as a s5miptom of reduced 

 precocity, a reduced contraction of the chromosomes at metaphase. 



