MEIOSIS 15 



of a number of small granules of chromatin of varying size, tlie chromo- 

 meres, embedded in an achromatic matrix. Very frequently homologous 

 chromosomes resemble one another very closely as regards the arrange- 

 ment and variations in size of the chromomeres which they contain. 



2. Meiotic or Reducing Division. 



A sexual process or syngamy, which consists in the union of two cells 

 together with fusion of their nuclei, occurs in higher animals and plants, 

 and it was amongst them that the nuclear changes associated with the 

 process were first studied. Attention has been drawn to the fact that 

 the chromosome number for each individual species is constant, so that 

 it must be evident that, if the nuclei of two cells unite, the number of 

 chromosomes in the resulting zygote nucleus, which is known as the 

 synkarion, would be double the usual number. This increase in number 

 does not actually occur, for the nuclei of the uniting cells or gametes 

 contain only half the number of chromosomes possessed by other cells. 

 The reduction is brought about by a special type of mitotic division of 

 the nucleus during the formation of the gametes (Fig. 4). When the 

 chromosomes arrange themselves on the spindle fibres as the equatorial 

 plate, instead of splitting into daughter chromosomes as in ordinary 

 mitosis, they become separated into two groups, one of each pair of 

 homologous chromosomes passing to each group (Fig. 4, C and D). In 

 this way the daughter nuclei contain half the number of chromosomes 

 possessed by the parent nucleus. The reduction in the number of chromo- 

 somes in the nuclei of the gametes is effected either at the last cell division 

 which gives rise to gametes, or at the one immediately preceding it. The 

 process is known as meiosis, and the nuclear division the meiotic division 

 or reducing division. When the gametes unite and their nuclei fuse, the 

 synkarion therefore contains the usual number of chromosomes. The 

 gamete with half the number of chromosomes is said to be haploid as 

 regards its chromosomes, while the original cell from which the gametes 

 were derived and the zygote resulting from their union, which contain 

 both chromosomes of each homologous pair, are said to be diploid. 



Amongst the higher animals, as also frequently amongst the Protozoa, 

 the gametes can be distinguished as male and female. The former, in 

 the vast majority of cases, are smaller than the latter, so that the gametes 

 can be distinguished as 7nicroga?netes and macrogametes. The micro- 

 gamete of a Metazoon is known as a spermatozoon and the macrogamete 

 as an ovum. The microgametes are derived from a large number of cells 

 called spermatogonia, which, like all the other cells of the body, contain 

 the normal or diploid number of chromosomes. One of these cells in- 



