REDUCTION OF CHROMOSOMES 



111 



sion does not enter in until the spores are being produced, 

 which give rise to the sexual generation (gametophyte). 

 This latter has the haploid number of chromosomes in 

 its nuclei. We must thus distinguish carefully between 

 typical asexual reproduction, where the resulting plant 

 is, as it were, but a separated part of the mother plant, 

 and the formation of a gametophytic generation from 

 the spore produced in the sporophytic generation. In- 

 deed each of these generations may have typical asexual 

 reproduction leading simply to the formation of other 

 plants of the same generation. 



160. After the union of gametes the chromosomes 

 from the two gametes remain separate, but usually the 

 corresponding chromosomes from each gamete lie close 

 together. In the reduction division the chromosomes 

 gather at the equator of the spindle as double chromo- 

 somes, in all probability representing the two corre- 

 sponding chromosomes from the two gametes. Before 

 this stage is reached, and while the chromatin matter 

 is found on fine 

 threads, there is a 

 characteristic bunch- 

 ing together of these 

 threads (called the 

 synapsis) in the course 

 of which it is sup- 

 posed that certain 

 characters become ex- 

 changed in the corres- 

 ponding chromo- 

 somes. These double chromosomes split apart and as 

 single ones go to the opposite poles. There are thus 

 entering into each daughter nucleus only as many chromo- 

 somes as were originally present in the gametes. These 



FIG. 49. Reduction division (diagrammatic). 



