Page:Anatomy of the Human Body (1918).djvu/42

42 are produced, viz., the mature ovum and three polar bodies, each of which contains two chromosomes, i. e., one-half the number present in the nuclei of the somatic cells of members of the same species. The nucleus of the mature ovum is termed the female pronucleus.

The spermatozoa or male germ cells are developed in the testes and are present in enormous numbers in the seminal fluid. Each consists of a small but greatly modified cell. The human spermatozoon possesses a head, a neck, a connecting, piece or body, and a tail (Fig. 6).

Fig. 6.—Human spermatozoon. Diagrammatic. A. Surface view. B. Profile view. In C the head, neck, and connecting piece are more highly magnified.

The head is oval or elliptical, but flattened, so that when viewed in profile it is pear-shaped. Its anterior two-thirds are covered by a layer of modified proto-plasm, which is named the head-cap. This, in some animals, e. g., the salamander, is prolonged into a barbed spear-like process or perforator, which probably facilitates the entrance of the spermatozoön into the ovum. The posterior part of the head exhibits an affinity for certain reagents, and presents a transversely striated appearance, being crossed by three or four dark bands. In some animals a central rod-like filament extends forward for about two-thirds of the length of the head, while in others a rounded body is seen near its center. The head contains a mass of