I] 



THE CHROMOSOME-CYCLE 



!3 



recovers, loops of the chromatin extending towards the nuclear wall {b). 

 The chromosomes then separate and contract, but their number cannot be 

 exactly estimated in sections at this stage {c). In the metaphase which 

 follows (d) the chromosomes, which are now bivalent by pairing, arrange 

 themselves to form the equatorial plate, and when this is seen from the polar 

 aspect, their number can be counted as 64, that is the haploid or reduced 





^f.^'r^ 



''"^-: '^'■'', 



^%^i 



•i* 



^^: 



mi. 





fe 



(J 



Fig. 30. Tetrad-division, with reduction of chromosomes in Dryopteris, 

 after Yamanouchi. a = nucleus with chromatin-reticuUim: (5 = nucleus 

 emerging from synapsis : ^ = nucleus approaching the metaphase: «'= early 

 metaphase, the bivalent chromosomes arranged in an equatorial plate: 

 £;= polar view of early metaphase showing 64 chromosomes, that is the 

 reduced number :/= early telophase, the daughter-chromosomes grouped 

 at either pole: two are delayed: ^= spore-mother-cell divided into two 

 hemispheres by a granular zone: each contains a daughter-nucleus: 

 /= polar view showing bivalent chromosomes: /= polar view of daughter- 

 chromosomes of the second division, showing their number to be 64 : 

 Z' = the second division completed, the nuclei reconstituted, and the cell- 

 plate dividing the two future spores of the half-tetrad from one another. 



number {e). The paired chromosomes then separate and group themselves 

 in the telophase at the poles of the spindle (/), constituting the new nuclei, 

 each of which will accordingly contain the haploid or reduced number {g). 

 Each of these nuclei then undergoes a further division (/?), and views of this 

 from the polar aspect show that the number of chromosomes involved is 64 

 {ij). Accordingly the nuclei formed from them, which become the definitive 



