CHAPTER XXII 

 REPRODUCTION IN THE METAZOA 



The general subject of reproduction was introduced in Chap. X, 

 and reproduction in Protozoa has been considered especially in Sees. 98, 

 104, 112, and 113. 



129. Methods of Reproduction in Metazoa. — In Metazoa the usual 

 type of reproduction is sexual by germinal cells, although asexual repro- 

 duction, by somatic cells, is found in the lower forms. Fission occurs 

 when the animal's body divides into two individuals equal in size. The 

 process is called budding when an individual gives rise to another by the 

 separation of a part smaller than that which remains and which is the 

 parent. Both fission and budding occur in many of the lower metazoans. 

 Some of the lower worms also undergo what is called fragmentation. 

 Though not the same as sporulation, in the sense in which the word is used 

 in connection with Protozoa, fragmentation is a mode of reproduction 

 analogous to it and occurs when the body divides into a large number of 

 fragments each one of which becomes a complete individual. 



130. Sexual Reproduction. — Sexual reproduction in Metazoa usually 

 involves two parents. It is then termed hiparental. In this case the two 

 parents usually differ from each other in their external appearance. 

 The one which is termed the male produces sperm cells; and the other, 

 called the female, produces egg cells. A species of which this is true is 

 termed diecious, or bisexual, referring to its existence in the two sexes. 

 On the other hand, particularly among lower Metazoa, there are those 

 species in which one individual produces both egg cells and sperm cells 

 and which therefore contains the organs of both sexes. Such species of 

 animals are represented by only one type of individual and are called 

 monecious, or hermaphroditic. Different species of hydra and of earth- 

 worms are examples of monecious animals; almost without exception the 

 vertebrates are diecious. 



131. Uniparental Reproduction. — Another method of sexual repro- 

 duction is for an egg cell to develop without union with a sperm cell. 

 When this reproduction by females alone takes place, the phenomenon is 

 termed -parthenogenesis. It occurs in nature in a number of diecious 

 animals in which exceedingly rapid reproduction contributes to the 

 welfare of the race. Examples of such animals are plant lice, which are 

 eaten by a vast number of other animals and which continue to exist only 

 by virtue of exceedingly great powers of reproduction, and certain 



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