on indefinitely--and these qualities cannot come down except through the chromosomes. The germ-plasm has come to us from the first man, and it will be passed on to the last person of the race--we are all literally uterine brothers. In the reduction of the germ-cells, if the primordial cell that finally produces the ovum has, say, four chromosomes, these four chromosomes first split longitudinally and reduce into two tetrads, or two groups of four chromosomes. Outside the nucleus is a spindle toward which the two tetrads move; they pass out of the nucleus and become the equatorial plane of the spindle; each tetrad divides into dyads (pairs of chromosomes), and one pair of these dyads remains in the ovum, while the other pair leaves the ovum entirely and becomes the nucleus of an abortive cell, called the First Polar Body. Later a second polar body forms and carries another dyad (two chromosomes) out of the ovum, leaving only one dyad, or two chromosomes, in the germ-cells; that is, half the number of chromosomes that were in the primordial cell. The reduction-division in spermatozoa is similar, but the end process leaves four active spermatozoa, whereas in the ovum the final result is one ovum and three practically inert and cast-off polar bodies. The reduction-division in both ovum and spermatozoön is in reality far more complicated than the broad summary given here. In parthenogenetic insects and animals a polar body takes the place of the spermatozoön, and fuses with the egg-nucleus to start mitosis. In general, the new nuclei in the cells formed by division are not made _de novo_, but arise from the splitting of the nucleus in the mother-cell. The new nucleus assimilates material, grows to maturity, and divides again into two daughter-nuclei. Whatever be the number of chromosomes that enter a new nucleus as it forms, the same number issues from it in mitosis. Boveri said,[31] "We may identify every chromatic element arising from a resting nucleus with a definite element that enters into the formation of that nucleus, from which the remarkable conclusion follows that in all cells derived in the regular course of division from the fertilized egg, one half of the chromosomes are of strictly paternal origin, the other half of maternal." It is not strictly true to say that the germ-nuclei fuse: they send in two sets of chromosomes that lie side by side, as has been frequently demonstrated since 1892[32] in many of the lower forms of life, and this law almost certainly extends also to man. [31] _Jenaische Zeitschrift_, 1891, p. 410. [32] See Wilson, _op. cit._, p. 299.