468 BOTANY OF THE LIVING PLANT 



4. The volume and amount of nuclear sap are now diminished. Several 

 conical groups of fibre-like lines, of which three are seen in the section, pass 

 from the nuclear membrane into the cytoplasm. The chromosomes are now 

 short and thick, and appear in pairs, the individuals of each pair being more or 

 less spirally wound upon one other. There is no recognisable nucleolus. 



5. The nuclear volume is further decreased, and nuclear sap is no longer 

 recognisable between the now closely grouped chromosomes, around which 

 the nuclear membrane is closely drawn ; meanwhile the fibre-like lines already 

 mentioned are aggregated towards several poles. 



6. Two such poles are here prominent, forming a bipolar spindle. The 

 nuclear membrane is no longer recognisable. From the tangle of paired 

 chromosomes, which marked the preceding stage, sixteen short dense chromo- 

 somes have emerged, w^hich are placed equatorially, and are segregated into two 

 groups each consisting of eight. 



7. Eight chromosomes have now passed towards each pole of the spindle. 

 Each group leads to the organisation of a fresh nucleus, with distinct nuclear 

 membrane, and with the eight chromosomes interwoven, but still clearly 

 recognisable. Between the two nuclei stretches a system of spindle-fibres. 



8. The fibres have disappeared. Nuclear sap is present between the 

 chromosomes in both nuclei. Since sixteen chromosomes were involved in 

 the organisation of the original nucleus, and each of the two daughter nuclei 

 has only eight chromosomes, this first division is a heterotype, or reduction- 

 division, the number in each being reduced to a half. The process is styled 

 Meiosis. 



9. Each daughter nucleus s here undergoing a second division. In each 

 case the nuclear membrane and the sap have disappeared, and a bi-polar 

 spindle has been organised around the eight chromosomes, arranged again 

 equatorially. Each chromosome has halved longitudinally, and a group of eight 

 chromosomes is moving towards each spindle-pole. This second division is a 

 somatic, or homotype division. 



10. The spindles have disappeared, and each group of chromosomes is 

 organising as a distinct nucleus. Between the nuclei lines of cleavage, at 

 right-angles to one another, indicate the di\dsion of the mother-cell into four 

 daughter-cells. 



11. Each nucleus has now a distinct membrane, and nuclear sap. The 

 chromosomes are still more or less distinct. Around each nucleus a definite 

 cytoplasm has been organised, and is enclosed within its own spore-wall. 



12. The tetrad of spores thus formed lies freely within the spore-mother- , 

 cell. Each nucleus has passed into the reticulate stage, with one or more ■ 

 nucleoli. As a consequence of the reduction each nucleus is now haploid, that is, 

 it has only eight in place of the original sixteen chromosomes of the spore- ; 

 mother-cell. | 



The critical point in the process of tetrad-division in its bearing; 

 on Sex and Heredity is in the stage preceding its first nuclear divi-; 

 sion (Fig. 393, III.-V.). ]\Iany detailed observations have shown thatj 

 as they emerge from " synapsis " the original chromosomes arcj 

 paired, lying with their longer axes parallel, or closely coiled 

 together. Subsequently the members of each pair separate, one^ 



I 



