MEIOSIS. — M. (a term introduced by Farmer and Moore in 1905) is the mechanism by which the chromosomes (v. CROMOSOMA) are reduced from the diploid condition (2n), characteristic of all the cells of the individuals of a species, to the haploid condition (n) of mature gametes (or sex cells): spermatozoa and ova (v. FECONDAZIONE; GAMETI E GAMETOGENESI). This haploid condition is necessary so that, through the union of the spermatozoon with the ovum at fertilization, the diploid number characteristic of the species may be restored.
M. was illustrated by Van Beneden (1809–94) and by Boveri in the maturation of the gametes of a nematode worm, a parasite of the intestines of horses, ascaris megalocephala, material that proved particularly suitable for this observation because of its small number of chromosomes (2n = 4 in the bivalens variety; 2n = 2 in the univalens variety). Nevertheless, the intimate essence of the process, characterized by the pairing of homologous chromosomes of maternal and paternal origin, was clarified only later through the studies of the Schreiner brothers (1905) in an annelid (tomopteris), of Grégoire in plants, of Agar in lepidosiren, of Janssens in urodeles, of Winivartier in the oogenesis of many species, and through the more recent observations of Darlington, White, and many others.
M. begins at the stage of the oocyte (in the female series) or spermatocyte (in the male series), which, at the end of their growth, are called first-order cells. This cell then undergoes two successive divisions, resulting in the formation of four cells; in the male line the four cells are all equal (spermatids) and will be transformed into four spermatozoa capable of fertilization, whereas in the female line one is large and filled with yolk, the ovum, and three are very small and without yolk and degenerate (polar bodies or polocytes).
But, whereas two cytokineses, that is, two divisions of the cell body, occur from the oocyte or spermatocyte stage, there is only one karyokinesis, that is, division of the nucleus, characterized by the splitting of the chromosomes into twice as many chromatids. In normal division, or mitosis, these are distributed between the two daughter cells (which therefore retain the same initial number of elements); here, however, they are distributed among the four daughter cells, each of which consequently comes to possess half the normal number. This distribution occurs in the second cytokinesis, whereas in the first the chromosomes, split into their two chromatids, pair (the homologues of maternal and paternal origin), then separate again and are distributed among the first two cells produced by the first division. The moment of pairing, which occurs during the diplotene and diakinesis phases, is particularly important, since, according to Morgan’s conception of crossing-over, it is assumed that exchanges, more or less numerous and extensive, of segments take place between the maternal and paternal homologues. The pairs of homologues, already split into individual chromatids, are called tetrads.
The diagram (v. fig) illustrates this process, as can be inferred from the spermatogenesis of grasshoppers, where, since the various chromosomes are of different lengths, the pairing of the homologues is more evident.
M., considered as a whole and in its particulars of the union and exchange of maternal and paternal homologous chromosomes, is of considerable importance for chromosomal conceptions of heredity (v. GENETICA).