Heredity and sex . but one that is unique in two , one of the cells is extremely small, as seen inFig. 19. The smallness is due to the minute amountof protoplasm that it contains. Second, the number ofchromosomes at each division is the half or ^haploid number. There is much evidence to show that at oneor at the other of these two divisions the two chromo-somes that had earlier united are separated, and in thisrespect this division differs from all other consequence, the egg nucleus, that re-forms after thesecond polar body has been produced, contains onlyhalf t
Heredity and sex . but one that is unique in two , one of the cells is extremely small, as seen inFig. 19. The smallness is due to the minute amountof protoplasm that it contains. Second, the number ofchromosomes at each division is the half or ^haploid number. There is much evidence to show that at oneor at the other of these two divisions the two chromo-somes that had earlier united are separated, and in thisrespect this division differs from all other consequence, the egg nucleus, that re-forms after thesecond polar body has been produced, contains onlyhalf the actual number of chromosomes characteristicof all the other cells of the female. In the formation of the spermatozoa a process takesplace almost identical with the process just described 38 HEREDITY AND SEX for the female (Fig. 20). In their earher history thegerm-cells of the male divide with the full number ofchromosomes characteristic of the male, which may beone less chromosome than in the female. The early. Fig. 20. — A-B, somatic cell division with four chromosomes. C-H,the two maturation divisions to produce the four cells {H) that becomespermatozoa. (After Wilson.) germ-cells then cease to divide for a time, and beginto grow, laying up yolk and other materials. At thistime the chromosomes unite in pairs, so that the num-ber appears to be reduced to half. Later two divisionsoccur (Fig. 20, D-H), in one of which the united chro-mosomes separate. The male germ-cells differ, how- THE MECHANISM OF SEX-DETERMINATION 39 ever, from the female, in that at each of these two di-visions the cells are equal in size. Thus four sperm-cells are produced from each original cell, all four pro-duce tails, and become spermatozoa. At the time of fertilization, when the spermatozoontouches the surface of the egg, the egg pushes out a coneof protoplasm at the point of contact (Fig. 19), and,lending a helping hand, as it were, to the sperm, drawsit into the egg. The projecting cone of pro
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Keywords: ., boo, bookcentury1900, bookdecade1910, booksubjectsex, bookyear1913
