Identifier: firstprincipleso00herb (find matches)
Title: The first principles of heredity; with 75 illustrations and diagrms
Year: 1910 (1910s)
Authors: Herbert, S. (Solomon), b. 1874
Subjects: Heredity
Publisher: London, A. and C. Black
Contributing Library: NCSU Libraries
Digitizing Sponsor: NCSU Libraries

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ulate hisown now generally accepted view, that the matura-tion of the ovum has no other purpose than to effectthe reduction of the chromosomes to half their originalnumber. But this reducing division is not so simple as justsuggested, because, before the reduction of the chromo-somes, a doubling of them first occurs, so that, in order toget ultimately the reduced number, the division has totake place twice. We have, to take an instance, an egg-cell with fourchromosomes before maturation. This egg-cell, as itripens, grows larger, and doubles its number of chromo-somes, having now eight instead of four. Now, in thefirst instance, half the number of these eight chromosomes—i.e., four—are removed from the mother egg-cell alto-gether by a process of division, as described above, under 38 THE FIRST PRINCIPLES OF HEREDITY the heading of Karyokinesis. But here there are to benoted two differences from the ordinary division. Firstly,the two daughter-cells resulting from the division are of
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Fig. 27.—Maturation of Ovum. (From Weismann, The Evolution Theory.) A, primitive ovum with four chromosomes ; B, mother-egg cell witheight chromosomes ; C, first maturation-division; D, formationof first polar body (2) ; E, second maturation division anddivision of first polar body into two (2 and 3) ; F, formationof second polar body (4). very unequal size, the larger one remaining as the ovumproper, while the smaller one forms the first polar body,which thus comes to lie outside the ovum. The second THE GERM-CELLS ^9 o difference is this : that each of the two daughter-cells doesnot receive, as happens in the regular Karyokinesis, thesame number of chromosomes as the mother-cell. Indeed,no splitting up of the chromosomes lengthwise takes placefor that purpose, but each daughter-cell receives only halfthe original number of chromosomes—i.e., in our case, asthe ripe ovum had eight chromosomes, the ovum, as wellas the first polar body, have after the first division four

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