THE PROTOZOA 



grow to full size before the next division. Under favorable conditions there 

 may be as many as 4 divisions, with the production of 16 individuals, in 24 

 hours. The rate of reproduction is determined by external conditions, such as 

 food and temperature, and by internal factors. 



Although Paramecium aurelia and probably other species of the same genus 

 may live indefinitely without conjugation, this process apparently occurs 

 under natural conditions as well as in the laboratory. In some cultures 



A. Two individuals unite by buccal grooves. 

 The micronuclei separate from the 

 macronuclei. 



B. The macronucleus begins to degenerate. 

 The micronucleus divides. 



C. The micronuclei divide again. Three 

 of each four disappear. 



D. The remaining micronuclei divide to 

 form migratory and stationary nuclei. 

 Exchange of migratory nuclei. 



E. The migratory and stationary nuclei 

 unite. 



F. The fusion nucleus is thus formed. 



G. The individuals separate. 



H. Division of the fusion nucleus. 



/. Division, as shown. 



J. Differentiation into macro- and micronuclei 

 and disappearance of three micronuclei. 



K. Cells and nuclei divide 

 as shown to produce the 

 original condition. 



Fig. 8.23. Parameaum: sequence of events in conjugation and subsequent divisions. (Adapted 

 from H. S. Jennings, Life and Death, Heredity and Evolution in Unicellular Organisms, copyright 

 1920 by R. G. Badger, printed by permission of Chapman and Grimes, Inc.) 



255 



