The Life History of Tillina Magna 413 



the beginning to the end of the history there has been a slow 

 decHne in the division rate. This was not noticed during the first 

 eight months, yet the diagram shows its presence. At the end 

 of eight months, a stimulus seemed necessary and was given in 

 K2HPO4. This caused a slightly higher rate of division, but it 

 could hardly be spoken of as having caused "rejuvenation, " for 

 almost immediately the vitality diminished, and a second, and soon 

 a third stimulus was needed, this exhaustion appearing more and 

 more frequently as the end drew near, and the stimulants having 

 less and less effect on the protoplasm, finally failing absolutely 

 in their potency. 



From these observations I am convinced that an artificial 

 rejuvenation of the protoplasm, in the sense of Calkins and Wood- 

 ruff, has not taken place at any time in the history of Tillina 

 magna, and that the effects of the stimulants have been to pro- 

 long rather than to renew life. 



But after all, is it not a question as to the meaning of "Artificial 

 Rejuvenation.?" According to Calkins and Woodruff, this term 

 has been applied to protoplasmic changes induced by chemical 

 or mechanical means, which result in a reorganization of the body 

 indicated by a renewal of metabolic processes and a high division 

 rate. A marked change in the protoplasm of Tillina has not 

 taken place after treatment with salts. Nevertheless, some action 

 occurred which enabled the stimulated culture to hold its own, 

 while the non-stimulated cultures died. Is this not practically 

 the same, only to a lesser degree, as that which Calkins and Wood- 

 ruff found in their so-called artificial rejuvenation of Paramecium 

 and Oxytricha .'' 



During certain periods in the life history, the vegetative activ- 

 ities of the organisms become exhausted and "physiological death" 

 (Hertwig's term) follows, unless some stimulus is given to renew 

 the vitality of the protoplasm. In such a conditfon, Tillina shows 

 but a slight degree of sensitiveness in its response to the treatment 

 with beef and potassium phosphate. Death is averted and the 

 organism is enabled to hold its own during the period of low vital- 

 ity. Oxytricha, in a similar period of vegetative exhaustion, 

 responds to a greater degree to stimulation, not only is death pre- 



