CHAPTER XVI 



MEIOSIS 



No subject in the field of cytology is of greater importance than 

 meiosis. This is mainly because the meiotic process affords a key to the 

 explanation of many phenomena of development and inheritance which 

 would otherwise be quite unintelligible ; in fact, the modern study of the 

 role of chromosomes in heredity has largely centered about changes in 

 nuclear constitution which occur at the meiotic period. Meiosis may 

 therefore be taken as the starting point in our discussion of cytogenetics, 

 which will be begun in the next chapter. 



As pointed out at the conclusion of the preceding chapter, each somatic 

 nucleus in ordinary diploid plants and animals contains a complement of 

 chromosomes made up of two intermingled sets descended by division 

 from those brought together in the previous union of gametes. In meiosis 

 there is accomplished what has long been referred to as "the reduction of 

 the chromosomes." This expression has had two meanings. In the 

 first place, it has meant the change from the zygotic to the gametic 

 number which occurs at this time; this is haplosis, or "numerical reduc- 

 tion." In the second place, it has been applied more specifically to the 

 disjunction of the two members of each pair of corresponding ("homolo- 

 gous") chromosomes present in the diploid complement, an event which 

 constitutes the central feature of meiosis. 



The Stage in the Life Cycle at Which Meiosis Occurs. — Haplosis 

 and chromosome disjunction are accomplished during the course of two 

 nuclear divisions known as the meiotic divisions. Because of its peculiar 

 appearance the first of these mitoses was termed "heterotypic" by 

 Flemming (1887), while the second, which was seen to resemble a somatic 

 division in certain features, was called "homoeotypic." Advances in our 

 knowledge of meiosis have led most cytologists to drop these terms and 

 to refer simply to the "first and second meiotic divisions." They are 

 also called "maturation divisions," especially in the zoological literature. 

 Since meiosis involves two divisions, the resulting nuclei or cells are 

 formed in groups of four, or quartets, although it frequently happens that 

 some members of the quartet do not function. For convenience the two 

 meiotic mitoses will be referred to frequently in subsequent pages as 

 I and //. 



In the normal life cycle of higher animals meiosis occurs at the time 

 of gametogenesis (Fig. 148). In the male the two divisions result in a 



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