CHAPTER VIII 

 MEIOSIS 



In all organisms reproducing sexually the doubling of the gametic 

 chromosome number (diplosis) by the union of the nuclei of two gametes 

 is compensated by a halving of the resulting zygotic number (haplosis) 

 at some other point in the life cycle. This quantitative alternation is in 

 itself a matter of considerable interest, but the full significance of the 

 changes involved can be appreciated only when one is aware of the peculiar 

 and orderly manner in which the reduction in number is accomplished 

 and of the effect that the alteration may have upon the capacities of the 

 nuclei that result. "Chromosome reduction" means not merely the 

 change from the zygotic to the gametic number, but also, more specificall}^, 

 the segregation (disjunction) of the two chromosomes composing each 

 homologous pair in the zygotic complement. 



These changes are brought about by two successive nuclear divisions 

 in the course of which the chromosomes are actually divided only once, 

 and the whole process constitutes meiosis. A nucleus undergoing meiosis 

 consequently gives rise to a c^uartet of nuclei, each of which has the 

 gametic chromosome number. Any cell in which meiosis is initiated 

 may be termed a meiocyte. In most animals the meiocj^tes are the pri- 

 mary spermatocytes, each of which produces a quartet of spermatozoa, 

 and the primary oocytes, each of which produces an egg and three (or two) 

 polar bodies. In most plants the meiocytes are sporocytes, each of which 

 produces a quartet of spores. 



In this chapter we shall give first a preparatory general account of the 

 behavior of the chromosomes -wdth special reference to their distribution to 

 the resulting four nuclei. This will be followed by a detailed account of 

 the changes occurring at each of the rather well-marked stages character- 

 istic of meiosis. For convenience the first and second meiotic divisions 

 will often be referred to simply as / and II. Only the ordinary diploid- 

 monoploid cycle will be considered here, discussion of chromosome behav- 

 ior in polyploid plants being deferred to a later chapter. 



Distribution of Chromosomes in the Meiotic Divisions. — Meiosis 

 begins in a nucleus with a diploid chromosome complement. The two 

 genomes were brought together at the previous gametic union; hence the 

 two chromosomes composing each homologous pair in the offspring of a 

 cross are derived one from each parent (see, however, page 216). The 



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