. A manual of elementary zoology . Zoology. io8 MANUAL OF ELEMENTARY ZOOLOGY has therefore the normal number, which it imparts to the body cells during the mitoses by which it joins them. Spermatogenesis takes place in the testis. Oogenesis begins in the ovary and finishes after fertilisation. The actual course of gametogenesis differs greatly in the two cases. In spermatogenesis the cytoplasm is equally divided and each of the products of division becomes a spermatozoon, so that from each spermatocyte four spermatozoa arise. In oogenesis each division is unequal. At the first division there a
. A manual of elementary zoology . Zoology. io8 MANUAL OF ELEMENTARY ZOOLOGY has therefore the normal number, which it imparts to the body cells during the mitoses by which it joins them. Spermatogenesis takes place in the testis. Oogenesis begins in the ovary and finishes after fertilisation. The actual course of gametogenesis differs greatly in the two cases. In spermatogenesis the cytoplasm is equally divided and each of the products of division becomes a spermatozoon, so that from each spermatocyte four spermatozoa arise. In oogenesis each division is unequal. At the first division there are formed a large cell and a small one, the latter containing very little cytoplasm and being known as the first polar body. At the second division, while the first polar body forms two very small cells,1 the large cell forms again a large and a small pro- duct. The large product is an ovum; the small is called the second polar body. Thus, instead of four ova the oocyte forms one ovum and three vestigial cells which come to nothing. The formation of the polar bodies is known as the maturation of the ovum. The meaning of these processes is very ob- scure. It is clear, however, (i) that, whatever be the reason for the occurrence of two divisions in gametogenesis, the fact of these being the same for both kinds of gametes makes the ovum and spermatozoon equivalent bodies, which contribute equally to the zygote nucleus ; (2) that the reduction division prevents the number of chromosomes from being increased by doubling at each conjugation; (3) that in this division the nuclear substance is not evenly divided by the splitting of each chromosome as in ordinary 1 In many animals the first polar body does not Fig. 67.—Sections of the egg of a frog during maturation and fertilisation.— Semi-diagrammatic. l, x, 3, Successive stages. ««, Nucleus of ovum; , , first and second polar bodies ; pig., pigmented protoplasm ; $ , female pronucleus ; 6 , male
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