The Reproductive System 253 



All animals have at least to some degree the power to regenerate 

 injured or lost tissues. Among some of the lower invertebrates this 

 power of regeneration is so great that it can be considered as a type of 

 asexual reproduction. In sponges, for example, fragments which are 

 broken ofif can easily develop into new individuals ; if a starfish is torn 

 in two in such a way that each part contains a portion of the central disc, 

 two new individuals will regenerate. 



While many animals reproduce both by sexual and asexual means, 

 the higher forms rely entirely upon sexual reproduction. Primarily, this 

 type of reproduction is of great advantage in providing a method for 

 new combinations of characters and a greater variety of forms. 



The protozoans show two types of sexual reproduction. Among the 

 ciliates (Paramecium, for example) two fuse temporarily and exchange 

 nuclear material. They then separate and produce new individuals by 

 binary fission. Conjugation is the name applied to this process. Other 

 protozoans have the more usual type in which two gametes fuse. This 

 is known as copulation or syngamy in contrast to conjugation. In some 

 species, the gametes are identical in size and appearance and are known 

 as isogametes ; in others, one gamete is much larger than the other. 

 These are known as heterogametes. All animals other than a few proto- 

 zoans reproduce by means of heterogametes. The large, nonmotile 

 gamete is the egg or ovum; the smaller motile gamete is the sperm. 



Among the vertebrates and the higher invertebrates, a single indi- 

 vidual is ordinarily able to produce only one type of gamete. In this 

 way, two individuals are absolutely necessary for reproduction. Often 

 these two are quite unlike in both appearance and behavior. The one 

 producing the eggs is the female; that producing the sperm is the male. 

 Animals of this type are said to be dioecious. Among many of the in- 

 vertebrates, a single individual produces both eggs and sperms. These 

 individuals are said to be hermaphroditic. In contrast to dioecious ani- 

 mals, these are also spoken of as being monoecious. Variation among 

 the offspring of monoecious individuals is often assured by cross fertili- 

 zation. 



Many other variations of sexual reproduction occur in different 

 animals. In some forms, only eggs are produced and these develop 

 without fertilization. This condition is known as parthenogenesis and is 

 common among many insects, crustaceans, and some snails. Often 

 parthenogenesis is found in these forms during periods of favorable 

 growth, while gametic union occurs before unfavorable conditions such 

 as cold weather appear. Experimentally, parthenogenesis has even been 



