CHAPTER II 



MEIOSIS 



In the life-cycle of the great majority of organisms there occurs a moment 

 when a new indi\ddual, the offspring, is formed by the fusion of two 

 reproductive cells budded off from the parents. The reproductive ceUs 

 are the gametes, and the cell formed by their union is the zygote. The 

 male gamete is the microgamete or spermatozoon, and the female gamete 

 the macrogamete or ovum. 



This periodical fusion of cells at fertilization or syngamy involves the 

 fusion of their nuclei, and hence a mechanism must exist to prevent a 

 corresponding periodical doubUng of the mass of nuclear constituents, or, 

 to put it from the point of \dew of the hypothesis of the continuity of 

 the chromosomes, we must look for a mechanism to prevent the doubling 

 of the number of the chromosomes at each act of syngamy. This 

 mechanism is found in the fact that each gamete is provided \\dth only 

 one-half of the number of chromosomes characteristic of the " species " 

 (i.e. the zygote or ordinary individual). ^ Thus the gamete is said to be 

 haploid and the zygote diploid in regard to their chromosome equipment. 

 The process of reducing the nimiber of chromosomes to one-half is known 

 as meiosis. 



The special cytological problems of the production of the gametes, 

 or gametogenesis, centre in the manner by which meiosis is brought about. 

 This always (in Metazoa) takes place in one of the last two mitoses 

 involved in the production of the gamete. Hence these two mitoses are 

 known collectively as the meiotic phase, though only one of them actually 

 effects the halving of the chromosome number, and is therefore, strictly 

 speaking, the meiotic division. In nearly all cases this division is the 

 first of the two mitoses of the meiotic phase — i.e. the penultimate mitosis 

 of that long series of divisions by which the gamete is produced from the 

 primordial germ cell. The second mitosis of the meiotic phase differs in 

 no essential from the ordinary mitoses of the body (somatic mitoses), 

 except that it takes place in a nucleus containing only half the number of 



1 This applies to all Metazoa. In a few Protista, and in many Metaphj'ta, the haploid 

 individual is the characteristic representative of the species, the diploid phase being very 

 transitory. See Chapter VII. 



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