CHAPTER IV 

 THE GENERAL OUTLINE OF MEIOSIS 



MEIOSIS is the antithesis of fertilization ; in 

 diploid organisms it results in the chromosomes 

 being reduced to the haploid number. If meiosis 

 takes place at the beginning of the life-cycle, just 

 after fertilization (this type of meiosis is called initial 

 or zygotic meiosis and occurs in most of the Sporozoa 

 and in the Charices,^? Basidiomycetes ^^ and Ascomy- 

 cetes ^^^ among plants as well as in some of the lower 

 Algae ^^) the synthesis (to use a philosophical term) 

 of fertilization and meiosis will be a haploid adult 

 organism. If on the other hand meiosis occurs just 

 before fertilization (i.e. during gamete formation) as 

 happens in all the higher animals (Metazoa) then the 

 adult organism will be diploid. In the higher plants 

 with an alternation between the sporophyte and 

 gametophyte generation meiosis takes place during 

 spore-formation, i.e. occupies an intermediate place in 

 the life -cycle ; where the gametophyte generation is 

 the predominant one (as in the mosses and liverworts) 

 the ' adult ' phase of the life -cycle will be haploid ; 

 where it is the sporophyte which is predominant (as 

 in the Pteridophytes and Phanerogams) the ' adult ' 

 phase will be diploid. Thus a moss-plant is haploid 

 and a buttercup diploid, but in both meiosis takes 

 place at the same stage in the life -cycle, during spore - 

 formation. 



Meiosis has been defined by Darlington ^^ as ' the 

 occurrence of two divisions of a nucleus accompanied 

 by one division of its chromosomes '. The whole 

 process must be regarded as having arisen in the 



47 



