Simultaneously with the forming of the two daughter nuclei the cell-body constricts across the middle of the somewhat elongated cell. The constriction increases until a complete division in the equatorial plane of the spindle has taken place. The result is the formation of two separate daughter cells (Fig. L). _Maturition._—This mode of cell division is made use of by the organism in the maturing of the germ cells into gametes. Before the conjugation of the ovum with the spermatozoön takes place, most ova extrude two polar bodies in the following way. Twice a division of the ovum takes place, each time into two quite unequal parts. The smaller, called polar bodies, remain near the periphery of the egg cell and are extruded later on. The same phenomenon is observed in the spermatocytes. The immature germ cell can neither impregnate nor become impregnated respectively, until, by a twice repeated cell division, a part of the nuclear substance has been cast off. Only after this has been effected does the germ cell become a gamete (a marriageable cell, from the Greek word γαμέω, I marry), i. e., a cell capable to impregnate or being impregnated. This maturition of the germ cell has hitherto been veiled in a mystery, which did not admit any plausible explanation. Why is the loss of half of the chromosomes necessary? Nowadays it has been accepted by most of the biologists that maturition stands in close connection with Mendel’s law of segregation. The learned monk Mendel, in his convent garden, made several important discoveries concerning the heredity of living organisms. He first discovered the quality which he called “unit characters”. Unit characters are, in the first place, the characteristics of the species, such as the number of the fingers. Unit characters are further characteristic of the sex, as the beard in the male and the breast in the female. Lastly there are the individual unit characters, as black hair or blue eyes. Unit characters are controlled by determiners which are either dominant or recessive. Unit characters, for this reason, do not blend. The color of the eyes, for example, is a unit character, and black is the dominant color. Hence when a black animal or plant is crossed with a white one, the hybrid is always black. The black type predominates in the influence upon the hybrid, while the type of white exercises the lesser influence. The second phenomenon Mendel discovered is that of segregation. By segregation is understood the separation of opposite determiners. Every unripe ovum or spermatocyte in the hybrid plant or animal, contains, for example, white and black determiners, but the ripe ovum or spermatozoön contains only one kind of determiner, either white or black, that is, during the ripening of the germ cell into the gamete one kind of the determiners has been eliminated.