[REDUCTION DIVISION SP ORE-FORMATION 307 



of each separating pair respectively represent the chromatic 

 material of the two parents from which the given organism arose 



(cf. p. 390)- 



The importance of this phenomenon is also emphasised by 

 the occurrence of an essentially similar method of reduction in 

 animals. Reduction occurs in the formation of spores, not only 

 in all Bryophyta and Pteridophyta, but also in the higher plants 

 (Phanerogams). In the case of the Thallophyta the actual reduc- 

 tion has only been observed in relatively few cases, but it cannot 



FIG. 171.- Nuclear divisions in spore mother-cells of Nephr odium (after 

 Yamanouchi). A, Late synapsis. B, Segmentation of chromosomes. 

 C, Association of chromosomes. D, Equatorial plate. E, Part of a 

 tetrad. 



be doubted that it takes place in all forms exhibiting a sexual 

 process. It would appear, however, that it may be effected at 

 various stages in the life-history, sometimes (as in Spirogyra) 

 during the first division in the zygospore, in other cases (e.g. 

 Fucus) during the formation of the sexual cells. It may be 

 noted that the latter is the customary stage for the occurrence 

 of the reduction division amongst animals. 



The four nuclei, produced in the spore mother-cell, usually 

 become grouped so as to form a tetrahedral figure. Each, 

 together with part of the cytoplasm, becomes surrounded by a 

 separate wall, the whole of the contents of the mother-cell being 



