Meiosis

MEIOSIS. - Meiosis (a term introduced by Farmer and Moore, 1905) is the mechanism of reduction of the chromosomes (v. CHROMOSOME) from the diploid condition (2n), characteristic of all the cells of the individuals of a species, to the haploid condition (n) of the mature gametes (or sex cells): spermatozoa and eggs (v. FERTILIZATION; GAMETES AND GAMETOGENESIS). This condition of haploidy is necessary because, with the union of the spermatozoon with the egg, in fertilization, the diploid number characteristic of the species is re-established.

Meiosis was illustrated by Van Beneden (1809-94) and by Boveri in the maturation of the gametes of a nematode worm, a parasite of the intestine of horses, ascaris megalocephala, a material that proved very favorable for this observation because of the small number of chromosomes (2n = 4 in the variety bivalens; 2n = 2 in the univalens). However, the intimate essence of the process, characterized by the pairing of the homologous chromosomes of maternal and paternal origin, was clarified only later with the studies of the brothers Schreiner (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 with the more recent observations of Darlington, White and many others.

Meiosis begins from the stage of oocyte (in the female line) or of spermatocyte (in the male line) which, at the end of their growth, are called of the I order. This cell therefore presents two successive divisions that lead to the formation of four cells; in the male line the four cells are all equal (spermatids) and will transform into four spermatozoa capable of fertilization, in the female line instead one turns out large and laden with yolk, the egg, and three very small ones, without yolk, which degenerate (polar bodies or polocytes).

But, while from the stage of oocyte or spermatocyte there are two cytokineses, that is two divisions of the cell body, there is only one karyokinesis, that is division of the nucleus, characterized by the splitting of the chromosomes into a double number of chromatids which, whereas in normal division, or mitosis, they are distributed into the two daughter cells (which therefore preserve the same initial number of elements), here are distributed into the four daughter cells, which therefore each come to have one half of the normal number. This distribution takes place in the second cytokinesis, while in the first the chromosomes, split into their two chromatids, pair (the homologues of maternal and paternal origin) and then separate again and distribute themselves into the first two cells produced by the first division. The moment of pairing, which takes place in the phases of diplotene and diakinesis, is particularly important since it is admitted, according to Morgan's conception of crossing-over, that more or less numerous and extensive exchanges of parts take place between the maternal and paternal homologues. The pairs of homologues, already split into the individual chromatids, are called tetrads.

The diagram (v. fig.) illustrates this process, as can be deduced from the spermatogenesis of grasshoppers, where, since the various chromosomes are of different length, the pairing of the homologues is more evident.

Meiosis, as a whole and in its particulars of union of the maternal and paternal homologous chromosomes and of exchange, assumes a notable value for the chromosomal conceptions of heredity (v. GENETICS).

Alberto Stefanelli