MEIOSIS. – Meiosis (a term introduced by Farmer and Moore, 1905) is the mechanism by which chromosomes (v. CHROMOSOME) are reduced from the diploid condition (2n), characteristic of all the cells of individuals of a species, to the haploid condition (n) of mature gametes (or sex cells): spermatozoa and ova (v. FERTILIZATION; GAMETES AND GAMETOGENESIS). This haploid condition is necessary so that, with the union of the spermatozoon with the ovum in fertilization, the diploid number characteristic of the species may be restored.
Meiosis was first described by Van Beneden (1809–94) and Boveri in the maturation of the gametes of a nematode worm, a parasite of the horse’s intestine, Ascaris megalocephala, a material that proved particularly favorable for this observation owing to the small number of chromosomes (2n = 4 in the bivalens variety; 2n = 2 in the univalens variety). However, 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 Winivarter in the oogenesis of many species, and through the more recent observations of Darlington, White, and many others.
Meiosis begins with the stage of the oocyte (in the female line) or of the spermatocyte (in the male line), which, at the end of their growth, are said to be of the first order. This cell then undergoes two successive divisions that result in the formation of four cells; in the male line all four cells are equal (spermatids) and will transform into four spermatozoa capable of fertilization, whereas in the female line one cell becomes large and yolk-laden (the ovum), and three become very small, yolk-free cells that degenerate (polar bodies or polocytes).
Whereas there are two cytodiereses, i.e., two divisions of the cell body, in the oocyte or spermatocyte stage, there is only one karyodieresis, i.e., a division of the nucleus, characterized by the splitting of the chromosomes into a double number of chromatids that, whereas in normal division, or mitosis, are distributed into the two daughter cells (which thus retain the same initial number of elements), here are distributed into the four daughter cells, each of which thus comes to have half the normal number. This distribution occurs in the second cytodieresis, whereas in the first the chromosomes, split into their two chromatids, pair (the homologues of maternal and paternal origin) and then separate again and are distributed into the first two cells produced by the first division. The moment of pairing, which occurs in the diplotene and diakinesis stages, is particularly important since, according to Morgan’s conception of crossing-over, more or less numerous and extensive exchanges of parts occur 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 it can be inferred from spermatogenesis in grasshoppers, where, since the various chromosomes are of different lengths, the pairing of the homologues is more evident.
Meiosis, in its entirety and in its details of the union of maternal and paternal homologous chromosomes and of exchange, assumes considerable significance for chromosomal conceptions of heredity (v. GENETICS).