CHAPTER 34 



X_JVERY species must reproduce itself or it 

 will soon disappear from the earth. Repro- 

 duction is one of the fundamental properties 

 of protoplasm and is exhibited by all ani- 

 mals, both one-celled and many-celled. The 

 theory of spontaneous generation that once 

 prevailed has given way to the concept that 

 all hfe arises from preexisting life. 



Reproduction and 

 Development 



ASEXUAL REPRODUCTION 



Binary fission in protozoans 



The simplest type of reproduction occurs 

 among one-celled animals and is known as 

 binary fission. This is a type of asexual repro- 

 duction because it is reproduction without 

 the intervention of sexual cells or gametes. In 

 certain species such as the amoeba, fission is 

 nothing more than cell division; but in the 

 more complex protozoans it may involve 

 working over the protoplasm of each daugh- 

 ter cell. For example, in the paramecium 

 both anterior and posterior daughters must 

 produce a new contractile vacuole, and a 

 new cytophar}'nx must be developed. Binary 

 fission may be transverse as in the Parame- 

 cium, or longitudinal as in the euglena. 

 Binary fission may also occur in cysts as in 

 the euglena. Multiple fission, which occurs 

 in many protozoans, especially among the 

 sporozoans, is usually a succession of binary 

 fissions within an envelope; but sometimes, 

 as in the malaria organism (Fig. 33), the 

 nucleus may divide a number of times, and 

 then the entire cell segments into as many 

 daughter cells as there are nuclei. 



558 



Fission in metazoans 



Among many-celled animals, fission can- 

 not be considered common. Planarians may 

 divide transversely, each part then becom- 

 ing organized into a normal animal. Any 

 missing parts of a new individual are 

 regenerated. Some annelids as well as certain 

 anthozoans may also reproduce in this way. 



