CHAPTER II. 

 THE GERM CELLS OVUM AND SPERMATOZOON 



It is customary, from the biologist's point of view, to divide the cells of 

 multicellular animals, or metazoa, into two classes: (i) the somatic cells and 

 (2) the germ cells. The somatic cells constitute the various tissues and organs 

 of the body and take part in the general physiological processes during the life 

 of the individual but perish without descendants when the individual dies. 

 The germ cells, on the other hand, are confined to the gonads, or genital glands, 

 play no role in the general economy of the individual; but are so specialized 

 that under proper conditions they give rise to a new individual and thus per- 

 petuate the species. 



In the entire vertebrate series of animals, and indeed in almost the whole 

 invertebrate series, the development of a new individual can take place only 

 after the union of two germ cells produced by two sexually different and mature 

 individuals. These cells are the egg (ovum, ovium) and the sperm (sperma- 

 tozoon, spermium), the former produced by the female, the latter by the male. 

 They are found in each sex in special glands the ovum in the ovary and the 

 spermatozoon in the testis from which they are detached at definite times 

 during sexual maturity. Prior to their union to form the starting point of a 

 new individual they pass through important preparatory stages which must 

 be considered along with their general characteristics. 



THE OVUM. 



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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