230 



ANIMAL BIOLOGY 



divisions known as multiple fission, or sporulation. This is 

 usually preceded by a considerable growth of the parent cell and 

 its enclosure in a protective covering, or cyst, which ruptures to 

 liberate the spores. Other unicellular forms, such as the Yeasts — 

 colorless plants chiefly responsible for alcoholic fermentation — 

 exhibit a modified form of fission in which the parent cell forms 

 one or several outgrowths, or buds, which gradually assume the 

 characteristic adult form and sooner or later become detached as 

 complete similar individuals. (Figs. 17, 25, 200, 223.) 



In a considerable number of instances, however, the cells arising 

 by multiple fission or budding remain closely associated or organi- 

 cally connected so that they form a colony. In some colonial 

 organisms the component cells are all alike and each retains its 



A. Paramecium. 



Cell Division 



(Binary fission) 



An indefinite number 



of generations. 



Temporary 

 Conjugation 

 (Fertilization) 

 Each cell fer- 

 tilizes the other. 



Cell Division Cell Division 



(Period of re- 

 construction) 

 Each fertilized 

 cell gives rise 

 to typical 

 animals. 



(Binary fission) 

 An indefinite 

 number of gen- 

 erations, 

 etc. 



B. Yolvox. 



g.c 



Cell Division Cell Division Cell Division Permanent Cell Division 



(Colony for- (Asexual re- (Gamete for- Conjuga- (Colony forma- 



production) mation) tion tion) 



Germ cells (g.c.) Certain germ (Fertilization) Zygote develops 



give rise to new cells produce One sperm into a colony, 

 colonies. eggs (e), others fuses with one etc. 



sperm (sp.). egg, forming 

 a zygote (z.). 



mation) 

 Zygote (z) de- 

 velops into a 

 colony. 



