PARAMECIUM 



137 



plate in an infusion in which the bacteria, having used up 

 the nourishment provided by the plant-remains, are falling 

 off in numbers, and thus the Paramecia, after a plentiful 

 supply of food, are beginning to experience dearth. But 

 there are some races in which it is difficult to bring 

 about conjugation, others in which it has never been seen, 

 and yet others in which it takes place at short intervals 

 without apparent cause. 1 



In a stock or "culture" of Paramecium kept in the 

 laboratory, it often happens that after a time 

 all the members pass into a state of "depres- 

 sion," in which they have an overgrown meganucleus and a 



meg 



Fig. 84. — Semidiagrammatic views of individuals of Paramecium 



caudalum. 



A , In depression ; B, in conjugation ; C, in fission. 



g., Gullet ; meg., meganucleus ; mi., micronucleus. 



stunted body, divide more slowly, and show an increasing 

 degeneration in various organs and functions of the body. 

 At last they are unable to digest food and die. Depression 

 has been regarded as the old age of the stock and compared 

 with the old age of the individual metazoan, but there are 

 some stocks in which it does not occur, and when it does 



1 It has been said that descendants of the same exconjugant will not 

 conjugate, and that individuals from another stock must be introduced, 

 but this has been disproved. 



