THE SEXUAL ELEMENTS OVUM AND SPERMATOZOON. 15 



rounding the axial filament is a thin cytoplasmic membrane or capsule 

 continuous with that of the body. In the human spermatozoon it is ap- 

 parently structureless; in other forms it assumes curious shapes as, for example, 

 the so-called membrana undulatoria, or wavy membrane of Amphibia, or the fine 

 membrane of some Insects.^ The terminal segment consists of the axial fila- 

 ment uncovered by any sheath. 



The significance of the various parts of the spermatozoon can be best 

 understood by reference to spermatogenesis (p. 17). 



Comparing the spermatozoon with a cell, the head contains the nucleus 

 while the body contains the centrosome. It is these parts of the spermatozoon 

 which are essential to fertilization. The acrosome and the tail may therefore 

 be considered as accessory structures which serve to bring and attach the 

 spermatozoon to the ovum. 



Within the tubule of the testis the spermatozoa show no evidence of motile 

 power. In the semen, however, which consists mainly of fluid secretions of 

 the accessory sexual glands, they move about freely, as also in the fluids of the 

 female genital tract. Their speed has been estimated at from 1.5 to 3.5 mm. 

 per minute and enables them to swim up through the uterus and oviduct, in 

 spite of the fact that the action of the cilia lining these tracts is against them. 



The life of the spermatozoon within the female genital tract is not known. 

 Moving spermatozoa have been found there seven to eight days after coitus. 

 In one case reported of removal of the tubes, living spermatozoa were found 

 three and one-half weeks after coitus. 



References for Further Study. 



COXKLIX, E. G.: Organ -forming Substances in the Eggs of Ascidians. Biol. Bull., 

 Vol. VIII, 1905. 



KEIBEL,-F. and MALL, F. P.: Manual of Human Embryology, 1910. Vol. I, Chap. I. 

 WALDEYER, W. : In Hertwig's Handbuch der vergleichenden u. experimentellen Entwick- 

 elungslehre der Wirbeltiere. Bd. I, Teil I, 1903. Also contains extensive bibliography. 

 WILSON, E. B.: The Cell in Development and Inheritance. 2d Ed., 1900. 



