62 



COLLEGE ZOOLOGY 



Micronucleus in mitosis, 

 macronucleus elongates 



\ 



Micronucleus begins mitosis 



y 



Micronucleus divides, macronucleus 

 pulls in two, new cell mouth and 

 two new contractile vacuoles appear 



Two daughter paramecia 



Figure 29. Division of Paramecium caudatum (binary fission). The first stage in the process 

 of division is shown at the top of diagram. 



scribed in Fig. 30. During conjugation there 

 is an interchange of micronuclei. The migra- 

 tory micronucleus (Fig. 30) is smaller than 

 the stationary micronucleus and may be 

 considered comparable to the nucleus of a 

 male germ cell. Its fusion with the stationary 

 micronucleus resembles the fusion of male 

 and female nuclei in the eggs of higher ani- 

 mals at the time of fertilization. Conjuga- 

 tion is similar to fertilization in that there 

 is a mixture of nuclear materials from two 

 individuals, and some authors consider the 

 fusion of micronuclei of conjugating individ- 

 uals as true fertilization. However, after con- 

 jugation the animals continue to reproduce 

 by asexual division in contrast to higher 

 animals in which there is only sexual repro- 

 duction. 



If the paramecia are kept in a constant 

 medium, for example, hay infusion, they 

 undergo a period of physiologic depression 



about every three months as shown by the 

 decrease in their rate of division. Semian- 

 nual periods also occur, and recovery from 

 these does not take place if the animals arc 

 kept under constant conditions, or conjuga- 

 tion is prevented; the protoplasm degener- 

 ates and becomes vacuolated, and the ani- 

 mals lose their energies and finally die. This 

 suggests that conjugation is essential for 

 continued asexual reproduction. 



Experiments have been performed on one 

 species which seem to show that in a varied 

 environment neither conjugation nor death 

 from old age necessarily occurs. Thus in one 

 experiment, a culture of Paramecium was 

 carried through a period of over 25 years 

 without the intervention of conjugation, by 

 changing the character of the medium daily. 

 During this time there were over 25,000 

 generations, and there was no evidence of a 

 decline in the vitality of the organisms as 



