Fig. 9
Diagram to illustrate spermatogenesis: a, showing the diploid number of chromosomes (six is arbitrarily chosen) as they occur in divisions of ordinary cells and spermatogonia; b, the pairing (synapsis) of corresponding mates in the primary spermatocyte preparatory to reduction; c, each secondary spermatocyte receives three, the haploid number of chromosomes; d, division of the secondary spermatocytes to form e, spermatids, which transform into f, spermatozoa.
Maturation of the Sperm-Cell.—In the maturation of the male gamete the germ-cell, now known as a spermatogonium, increases greatly in size to become a primary spermatocyte. In each primary spermatocyte the pairing of the chromosomes already alluded to occurs as indicated in Fig. 9b, [p. 42], where six is taken arbitrarily to indicate the ordinary or diploid number of chromosomes, and three the reduced or haploid number. The division of the primary spermatocyte gives rise to two secondary spermatocytes (c), the paired chromosomes separating in such a way that a member of each pair goes to each secondary spermatocyte. Each secondary spermatocyte (d) soon divides again into two spermatids (e), but in this second division the chromosomes each split lengthwise as in an ordinary division so that there is no further reduction. In some forms the reduction division occurs in the secondary spermatocytes instead of the primary. Each spermatid transforms into a mature spermatozoon (f). The spermatozoa of most animals are of linear form, each with a head, a middle-piece and a long vibratile tail which is used for locomotion. The head consists for the most part of the transformed nucleus and is consequently the part which bears the chromosomes.
Maturation of the Egg-Cell.—As regards the behavior of the chromosomes the maturation of the ovum parallels that of the sperm-cell. There are not so many primordial germ-cells formed and only one out of four of the ultimate cells becomes a functional egg. As in maturation of the sperm-cell there is a growth period in which oögonia enlarge to become primary oöcytes (Fig. 10b, [p. 45]). In each primary oöcyte as in the primary spermatocyte the chromosomes pair and two rapidly succeeding divisions follow in one of which the typical numerical reduction in the chromosomes occurs. A peculiarity in the maturation of the ovum is that there is a very unequal division of the cytoplasm in cell division so that three of the resulting cells usually termed polar bodies are very small and appear like minute buds on the side of the fourth or egg-cell proper.
The scheme of this formation of the polar bodies is indicated in Fig. 10, [p. 45]. In Fig. 10b the chromosomes are seen paired and ready for the first division; that is, for the formation of the first polar body. Figs. 10c, d, [p. 45], show the giving off of this body. Note that while only a small proportion of the cytoplasm passes into this tiny cell, its chromatin content is as great as that of the ovum. A second polar body (Figs. 10e, f, [p. 45]) is formed by the egg, but in this case each chromosome splits lengthwise, as in ordinary mitosis, and there is no further numerical reduction. In the meantime, typically, a third polar body is formed by division of the first. (Stages e, f, g.)
Parallel Between the Maturation of Sperm- and Egg-Cell.—This rather complex procedure of the germ-cells will be rendered more intelligible through a careful study of Figs. 9 and 10, [pp. 42] and [45], and Fig. 11, [p. 46], which indicates the parallel conditions in spermotogenesis and oögenesis.
Fig. 10
Diagram to illustrate oögenesis: a, showing the diploid number of chromosomes (six is arbitrarily chosen) as they occur in ordinary cells and oögonia; b, the pairing of corresponding mates preparatory to reduction; c, d, reduction division, giving off of first polar body; e, egg preparing to give off second polar body, first polar body ready for division; f, g, second polar body given off, division of first polar body completed. The egg nucleus, now known as the female pronucleus, and each body contain the reduced or haploid number of chromosomes.
The view now generally held regarding the polar bodies is that they are really abortive eggs. They later disappear, taking no part in embryo-formation. It can readily be seen how such an unequal division is advantageous to the large cell, for it receives all of the rich store of food material that would be distributed among the four cells if all were of equal size. This increased amount of food is a favorable provision for the forthcoming offspring whose nourishment is thus more thoroughly insured.