The union of two gametes, a sperm and an egg, is the essential fact of sex- 

 ual reproduction. It is called fertilization. 



In the vertebrates, eggs, discharged into the body cavity by the ovary, 

 pass through an oviduct before reaching the exterior; sperms, developed ii? 

 the testes, also pass through a special duct to the exterior. 



The distinctive variations in reproductive organs are related to the manner 

 in which eggs and sperms are brought together and to the way the fertilized 

 egg cell is nourished. 



In the more complex species the gonads — the ovary of the female and the 

 testes of the male — are both hormone-producing as well as gamete-producing 

 organs. 



Among mammals the embryo develops within the uterus until it attains a 

 form distinctive of the species. 



Many species of plants and animals produce vegetatively, as well as sexually. 



Some species of plants and animals reproduce alternately by vegetative 

 and sexual processes. 



Among insects, as among reptiles, birds and mammals, fertilization takes 

 place within the body of the mother. 



At all levels of animal life the male gamete, or sperm, is motile; the female 

 gamete, or egg cell, is passive and richly supplied with food. 



The number of chromosomes present in gametes is half that present in 

 body cells. 



Most familiar plants, as well as animals, reproduce by forming male and 

 female gametes, that is, sexually. 



Among lower forms of animal life there are hermaphroditic species, that is, 

 forms in which the individual bears both male and female gametes. 



Parallel to the differences between the gametes, males are characteristically 

 restless, roving, searching, aggressive; the females are passive, receptive, 

 eventually directing their resources to the nourishment of offspring. 



Characteristic differences between males and females which have no direct 

 connection with reproduction are spoken of as secondary sex characteristics. 



EXPLORATIONS AND PROJECTS 



1 To study reproduction in the ameba, examine several specimens mounted 

 under a cover-glass, with both low and high magnifications; watch for cell- 

 division. Sketch different stages in the division. Try to distinguish the nuclei 

 within. Describe the way the ameba reproduces. 



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