CHAPTER VIII 

 MEANS AND METHODS OF REPRODUCTION 



The reproduction of an individual organism begins with the 

 separation from the parent of a part of its body, which is of course 

 alive and continues to live as it progresses through developmental 

 stages to the adult condition. Reproduction is a method of con- 

 tinuing life rather than one of creating life. It results in the 

 production of generations of new individuals which, however, 

 are derived from the preceding generations through living 

 material. Single-celled animals, Protozoa, reproduce by the 

 division of the entire animal. This also occurs in some of the 

 lower metazoan animals. In all of the Metazoa, regardless of 

 the occurrence or not of reproduction by simple division of the 

 animal as a whole, there is a form of reproduction in which the 

 development of a new individual starts in an egg that is 

 usually fertilized by a spermatozoon. Both egg and sperma- 

 tozoon are cells, so that the morphology of the elements out of 

 which the new individual is produced is quite different from the 

 morphology of the adult animal. The organism in all trans- 

 formations undergone from the egg stage to the adult stage is a 

 living thing. Life does not start at the hatching of the frog 

 tadpole or at the birth of a human being. Life started in the far 

 distant past and has continued in those kinds of organisms 

 still alive at the present time. Extinct species are forms which 

 for one reason or another ceased to produce descendants. 



The organs of reproduction are the ovaries and testes, also 

 known as gonads. In many invertebrates both ovary and testis 

 are present in the same individual, in which case the animal is 

 said to be monoecious or hermaphroditic. Other invertebrates 

 and most vertebrates are dioecious, which means that two sexes 

 are present and that ovaries are present only in the female and 

 testes in the male. Sometimes, as in some of the Mollusca, 

 there is a single gonad, the ovotestis, one portion of which produces 

 eggs, the other spermatozoa; or the ovotestis may at one time 



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