CHAPTER 14 



THE REPRODUCTIVE SYSTEM 



For every species there are two fundamental necessities : one is the 

 maintenance of the individual, the other the continuation of the species 

 by the production of new individuals. All the systems thus far con- 

 sidered are primarily concerned with the first necessity ; the reproductive 

 system alone is concerned with the latter need. The reproductive sys- 

 tem, of course, is but one part of the whole organism ; thus it is depend- 

 ent for its functioning on the other organ systems. 



Among the higher animals, new individuals are ordinarily repro- 

 duced by the union of sex cells or gametes (eggs and sperm). Repro- 

 duction which involves gametes is known as sexual reproduction. In 

 contrast to this gametic reproduction is that in which a single individual 

 animal divides in some manner to produce offspring; this is known as 

 asexual reproduction and is common among many of the invertebrates. 



Asexual reproduction may be of several types. In its simplest form, 

 an animal divides into two new individuals, each of which is nearly iden- 

 tical in size and appearance. This type of reproduction, binary fission, 

 is most commonly seen among some of the protozoans. The division may 

 be either longitudinal or transverse, depending upon the species. Slightly 

 more complex is the mode of asexual reproduction known as budding. 

 In this method, the new individual is usually smaller than the parent 

 and may appear as an internal aggregate of cells or perhaps as a minia- 

 ture on the outer surface. Internal budding is very common among the 

 sponges; while many of the coelenterates (Hydra, for example) demon- 

 strate the external type. 



Some of the parasitic protozoans demonstrate another type of 

 asexual reproduction in which a single individual divides internally to 

 produce numerous progeny. This is known as multiple fission. Among 

 the flatworms, some forms very commonly reproduce by dividing trans- 

 versely to produce several new individuals. 



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