. The cell in development and inheritance. Cells. GENERAL OUTLIAE 175 in the most general terms, this parallel is as follows: ^ In both sexes the final reduction in the number of chromosomes is effected in the course of the last two cell-divisions by which the definitive germ-cells arise, each of the four cells thus formed having but half the usual number of chromosomes. In the female but one of the four cells forms the "ovum" proper, while the other three, known as t\\ bodies, are minute, rudimentary, and incapable of development (Figs. 64, 71, 86). In the male, on the other
. The cell in development and inheritance. Cells. GENERAL OUTLIAE 175 in the most general terms, this parallel is as follows: ^ In both sexes the final reduction in the number of chromosomes is effected in the course of the last two cell-divisions by which the definitive germ-cells arise, each of the four cells thus formed having but half the usual number of chromosomes. In the female but one of the four cells forms the "ovum" proper, while the other three, known as t\\ bodies, are minute, rudimentary, and incapable of development (Figs. 64, 71, 86). In the male, on the other hand, all four of the cells become functional spermatozoa. This difference between the two sexes is probably due to the physiological division of labour between the germ- cells, the spermatozoa being motile and very small, while the q^^ contains a large amount of protoplasm and yolk, out of which the Primordial germ-cell. Oogonia. Primary oocyte or ovarian egg. Secondary oocytes (egg and first polar body). ^Mature egg and three polar Division-period (the number of divi- sions is much greater). Growth-period. " Maturation-period. Fig. 87. — Diagram showing the genesis of the egg. [After Boaeri.] main mass of the embryonic body is formed. In the male, therefore, all of the four cells may become functional; in the female the func- tions of development have become restricted to but one of the four, while the others have become rudimentary (cf. p. 182). The polar bodies are therefore to be regarded as abortive eggs — a view first put forward by Mark in 1881, and ultimately adopted by nearly all inves- tigators. I. Reduction in tJie Female. Formation of the Polar Bodies As described in Chapter III., the Qgg arises by the division of cells descended from the primordial egg-cells of the maternal organism, and these may be differentiated from the somatic cells at a very early ^ The parallel was first clearly pointed out by Plainer in 1889, ^f^' was brilliantly demon- s
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectcells, bookyear1896
