3O6 L. L. WOODRUFF. 



its prevalence in Calkins' cultures on a constant medium. This 

 idea is not supported by my hypotrichous cultures which were 

 carried on a constant medium and still did not develop a tend- 

 ency to conjugate even when the culture medium was varied in 

 some special experiments. It is highly probable, however, that 

 the superficial conditions which induce conjugation may vary in 

 different species. 



Periods of marked physiological depression have not appeared 

 during the first twenty-six months of the life of the Paramecium 

 I culture, but well-defined morphological changes have taken 

 place. I shall not discuss these cytological changes at present, 

 as I believe it is advisable to wait until the culture is terminated 

 naturally, or by accident, so that all the data from the complete 

 series may be discussed in its entirety. It is clear, however, that 

 the relation of the rate of division to the so-called "normal" 

 condition of the nuclei of Paramecium is not supported by this 

 culture, as decided nuclear changes apparently do not affect the 

 general vitality of the organisms. It may be noted further, that 

 not a single monster due to incomplete or otherwise abnormal 

 division has occurred in the entire 1,238 generations. 



V. CONCLUSIONS. 



The experimental study of the life history of infusoria has so 

 far clearly shown that : 



The protoplasm of these organisms, when subjected to a com- 

 paratively constant culture medium, passes through long cyclical 

 changes in vitality which finally result in the death of the 



organism. 



The protoplasm may be "rejuvenated" by. suitable changes in 

 the culture medium (stimuli) at critical points in the cycle, and 

 thus be enabled to resume active reproduction for a longer 

 period. 



The essential fact brought out by this study is that : 

 The protoplasm of the individual Paramecium isolated over 

 two years ago to start the culture has had the potential to divide 

 (so far) over one thousand two hundred and thirty times at an 

 average rate of more than three divisions every two days, and 

 the representatives of the untold millions of its progeny which are 



