3o6 REDUCTION— DIVISION 



Prior to the first division the chromatin reticulum concentrates 

 into its constituent chromosomes, but these soon appear joined 

 togetlier to form a thread. The latter thereupon contracts in 

 a tangled manner around the nucleolus, towards one side of the 

 nuclear cavity (Fig- 171, A) ; this stage, which is termed synapsis, 

 has in other groups been recognised in living material, and is 

 not therefore a contraction due to methods of fixation. 



As the thread spreads out again, on emerging from this 

 condition, the nucleolus is seen to have disappeared, its chromatic 

 material having presumably been absorbed into the chromatin 

 thread. At the same time the latter becomes thrown into a 

 number of loops (Fig. 171, A), each formed of two, more or less 

 closely approximated, chromosomes, joined end to end. \'\'hen 

 segmentation takes place (Fig. 171, B), the number of separate 

 portions produced is only half that observed in the vegetative 

 divisions of the same plant, each segment being composed of 

 the two chromosomes forming one of the above-mentioned loops 

 (Fig. 171, C). 



The nuclear spindle is established in the ordinary way, and 

 the double chromosomes become grouped at its equator 

 (Fig. 171, D). Thereupon each chromosome splits transversely 

 into two, one half (actually a icJiole chromosome) passing to either 

 pole. Each of the daughter-nuclei thus contains half the number 

 of chromosomes characteristic of the vegetative cells, and, for 

 this reason, this nuclear division is often spoken of as the 

 " reduction " division. The second nuclear division follows almost 

 immediately on the first, and does not differ essentially from 

 that described for the ordinary vegetative cells.' 



Each of the four resulting spores (Fig. 171, E) thus possesses 

 the halved number of chromosomes, and so do all the cells of 

 the gametophyte arising from a spore. The normal number of 

 chromosomes is restored in the fertilised egg, in consequence of 

 the fusion of the male and female nuclei. It will be realised that 

 the reduction division differs chiefly from the normal t^'pe in 

 the occurrence of synapsis and in the passage of entire chromo- 

 somes to the respective poles of the nuclear spindle. Moreover, 

 there is good reason for believing that the associated chromosomes 



1 The phenomena exhibited during the two nuclear divisions in the spore 

 mother-cells are often referred to as meiosis. 



