CHAPTER 6 



Reproduction 



The processes needed for the day-to-day survival of the organism— nu- 

 trition, respiration, excretion, coordination, and the rest— were discussed 

 in the preceding chapter. The survival of the species as a whole requires 

 that its individual members multiply, that they produce new individuals 

 to replace the ones killed by predators, parasites or old age. One of the 

 fundamental tenets of biology, "omne vivum ex vivo" (all life comes only 

 from living things), is an expression of this basic characteristic of all 

 living things, their ability to reproduce their kind. 



For centuries it was believed that many animals could arise from 

 nonliving material by "spontaneous generation." For example, maggots 

 and flies were thought to originate from dead animals, and frogs and 

 rats to come from river mud. The classic experiments which disproved 

 the theory of spontaneous generation were performed by Francesco 

 Redi about 1670. By the simple expedient of placing a piece of meat 

 in each of three jars, leaving one uncovered, covering the second with 

 fine gauze and the third with parchment, he demonstrated that although 

 all three pieces of meat decayed, maggots appeared only on the un- 

 covered meat. Maggots do not come from decaying meat, but hatch 

 from eggs laid on the meat by blowflies. With the development of 

 lenses and microscopes, and the subsequent increase in knowledge of 

 eggs and larval forms, we now know that no animal arises by spontane- 

 ous generation. 



The process of reproduction varies tremendously from one kind of 

 animal to another, but we can distinguish two basic types: asexual and 

 sexual. In asexual reproduction a single parent splits, buds or fragments 

 to give rise to two or more offspring which have hereditary traits iden- 

 tical with those of the parent. Sexual reproduction involves two indi- 

 viduals; each supplies a specialized reproductive cell, a gamete. The 

 male gamete, the sperm, subsequently fuses with the female gamete, the 

 egg, to form the zygote or fertilized egg. The egg is typically large, 

 nonmotile, and contains yolk which supplies nutrients for the embryo 

 which results if the egg is fertilized. The sperm is typically much 

 smaller and motile, adapted to swim actively to the egg by the lashing 

 movements of its long, filamentous tail. Sexual reproduction is advan- 

 tageous biologically for it makes possible the recombination of the best 

 inherited characteristics of the two parents and provides for the possi- 



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