VII. SEXUAL REPRODUCTION AND REJUVENESCENCE 271 



sitic nature, they may have proper rejuvenation means like this Para- 

 mecium. 



In addition, certain organisms are apparently able to accomplish 

 rejuvenation to a certain extent merely by the change of environmen- 

 tal condition under which they grow. According to our finding (80), 

 when cultivated in broth for successive generations, typhoid bacilli 

 will be reduced in virulence, changed to R-type, and their resistance 

 to heat is diminished, becoming liable to die out, whereas when the 

 bacteria declined in such a way are transferred on agar-plate, they 

 will be changed into S-type, whereby their virulence is enhanced and 

 the resistance is increased. However, successive cultivation on the 

 same agar-plates also lead to a decline in the virulence. The sole 

 change in the temperature at which the bacteria are cultivated has a 

 similar effect, the virulence tending to increase when the temperature 

 is elevated over 37°C. 



The development of ascaris is accomplished when the eggs are 

 exposed to low temperatures outside the host body, a fact which may 

 also be an example that rejuvenation is established by the change of 

 environment without contact with another individuals. 



It can by no means be considered, however, that such a rejuve- 

 nation is perfect and complete. As is generally known, the enhance- 

 ment in the virulence of bacteria, which have been decayed by the 

 continuous cultivation under the same environmental condition, is 

 accomplished usually by passage through proper animals, a phenome- 

 non which must be analogous to the sexual conjugation as v.^e shall 

 see in the following section. 



2. Rejuvenescence of Microorganisms by Making Use of the 

 Sexual Reproduction of the Host 



As seen in the preceding Chapter, the deformation in the pattern 

 of viruses may arise if they continue to multiply in the protoplasm 

 of the same kind of organisms. This deformation will be repaired by 

 the host change with the achievement of the rejuvenation of the 

 virus. It is considered that the host change answers not only the 

 purpose of rejuvenation, but also the purpose of producing new 

 species, since viruses through the host change may be able to get 

 possession of the new protoplasm structure of the new host. The 

 host change, therefore, is comparable to the sexual reproduction. 



In many parasitic protozoa, for example, the parasitic amoebae and 

 the intestinal and blood flagellates, no sexual process has been obser- 



