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. 97, 

 103, 111, and 112. 



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

 type of reproduction is sexual, although asexual reproduction 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 sporu- 

 lation, 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. 



128. Sexual Reproduction. — Sexual reproduction in Metazoa usually 

 involves two parents. It is then termed hi-parental. 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. 



129. Uniparental Reproduction. — It is possible for an egg cell to 

 develop without union with a sperm cell. When this 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 aquatic forms, like rotifers and water fleas, which are also eaten 

 in great numbers by fish and other larger aquatic animals. 



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