CHAPTER II. 

 THE SEXUAL ELEMENTS— OVUM AND SPERMATOZOON. 



In practically the whole animal kingdom and without exception in the entire 

 vertebrate series the development of a new being can take place only when 

 reproductive elements, produced by two sexually different individuals, are 

 brought into union at the proper time as the result of the procreative act. 

 These elements are, on the one hand, the egg or ovum, which is produced by the 

 female, and, on the other hand, the seminal filament or spermatozoon produced 

 by the male. 



To a student of histology it is a matter of knowledge that the ovum and 

 spermatozoon are histological units or cells. It is a familiar fact, too, that 

 both are produced in special glandular organs — the ovum in the ovary of the 

 female and the spermatozoon in the testis of the male. Furthermore, it is 

 known that during sexual maturity they detach themselves from the sexual 

 organs at definite times. They then form under suitable conditions, among 

 which the union of the two sex cells is the most important, the starting point of 

 a new organism. As a necessary preliminary to the understanding of the 

 development which follows the union of the two sex cells, a brief consideration 

 of the structural and to a certain extent of the physiological peculiarities of 

 these cells is essential. 



THE OVUM. 



The ovum or egg cell is a cell specially organized for the perpetuation of the 

 species. It possesses those extraordinary peculiarities of growth and differ- 

 entiation possessed by no other cell, which enable it to give rise directly to a 

 new organism with all the characteristics of the species. Among Vertebrates, 

 however, the process of development is made possible only by union with the 

 male sexual element — the spermatozoon. It should be stated, moreover, that 

 the ovum as seen in a Graafian follicle in the ovary is not a true sexual element, 

 not having undergone certain processes which eliminate part of its chromatic 

 substance and put it on a par with the spermatozoon as a mature sexual ele- 

 ment (see page 27). The large egg cell in the ovary is properly called a 

 primary oocyte. 



With the exception of some neurones, the human ovum (Fig. 5) is the 

 largest cell in the body. It is spherical in shape, measuring from 0.15 mm. to 

 0.2 mm. in diameter, contains a large spherical nucleus and is surrounded by a 

 relatively thick, transparent membrane. As seen in section in the ovary it has 



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