Returning now to the origin of gametes. The spermatogonia and oogonia in the reproductive organs are, together with all the cells forming the body proper, direct descendants by mitotic cell division from the fertilized egg which gave rise to the individual organism. This is equally true of the chromosomes themselves and accordingly every cell of the animal has the same number of chromosomes as the fertilized egg.
Fertilization, as we now know, always consists of the fusion of two gametes, whether it is in plants or animals; a fusion of nucleus with nucleus and cytoplasm with cytoplasm to form a zygote, which therefore is one cell reconstructed from two. Such being the case, one of two things must happen at fertilization. Either the fertilized egg must have double the chromosome number, that is a set contributed by both egg arid sperm; or some method must exist by which the chromosomes of the gametes are reduced in number to one half that characteristic of the somatic cells.
How Gametes Work Video?
As a matter of fact a reduction in the number of chromosomes always takes place sometime during the life history.
In plants such as the Mosses, Ferns, and Flowering Plants, it occurs at the formation of the spores. Thus it follows that the gametophyte contains half as many chromosomes as the sporophyte, and the sporophyte number is restored by the union of the gametes. It must be borne in mind, however, that the familiar plants are sporophytes which, for all practical purposes, directly produce sporophytes because the gametophyte is reduced almost to the vanishing point. The chromosome number of the parent sporophyte and the sporophyte in the seed is the same. But we cannot digress To elaborate the details of the chromosome cycle associated with alternation of generations in plants attention must be concentrated on the conditions as they exist in animals, in which the somatic number of chromosomes is reduced one half at the formation of the gametes. From the standpoint of chromosome number, the sporophyte is comparable to the animal soma and the gametophyte is represented by merely a couple of cell generations during the formation of the gametes in animals.
The maturation or 'ripening' of the germ cells of animals involves two cell divisions by which each spermatogonium gives rise to four sperm, and each oogonium to one functional egg and three tiny, abortive eggs known as POLAR BODIES; each and all with one half the number of chromosomes of the somatic cells and of the germ cells up to this point in their development. Consequently these two divisions, termed MATURATION DIVISIONS, must be examined in some detail if we are to appreciate the nicety of the process by which the chromosome number is reduced one half without impairing the chromatin heritage from cell to cell. We shall describe first the origin of the sperm which, though it is fundamentally the same as that of the egg, is somewhat simpler to under stand.