EFFECTS OF THYROID ON PARAMAECIUM 553 



race may in some case's rejuvenate itself by a peculiar process of 

 parthenogenesis (endomixis) . 



The cyclical character of the life histories of my three races is 

 shown by figures 1, 2, and 3. A few cases where experiments 

 with thyroid were in progress at depression periods may now be 

 considered. Experiment A 5 was commenced shortly before the 

 beginning of a depression period (fig. 1) where for four days there 

 was but one individual in the control line. A line which had 

 been derived from this a few days previously and treated with 

 thyroid divided during this period at a rate of three and seven- 

 tenths divisions per day. This and other experiments have in- 

 dicated that initial stimulation with thyroid results in carry- 

 ing the race at a high rate of division through a depression period 

 from which the control line recovers with great difficulty if at 

 all. So in Experiment B 5 (fig. 5) during a similar depression 

 period the control line died out, while a line which had been re- 

 cently treated with boiled thyroid went through the depression 

 period with a division rate higher than normal. 



In this same experiment however a thyroid line which had 

 been under treatment for a considerable period died out at the 

 same time as the control. Referring to Experiment A 9, we find 

 that in the depression period that carried off the race, a thyroid 

 line of twenty days treatment, which at first had produced an 

 increase in division rate, succumbed at the same time as the 

 control. 



Comparing the life cycle curves of thyroid fedand control 

 lines (figs. 5 and 6), we note that in these two experiments, each 

 over three months in duration, the thyroid fed line, though 

 always at a much higher division rate shows the same rhythms 

 as the control. Much of this similarity is of course due to simi- 

 lar environmental causes, changes in temperature, etc. But it is 

 very striking that although the thyroid line is many generations 

 older than the control line, the periods of depression occur at the 

 same times. These experiments further indicate that the maxi- 

 mum effects of the thyroid treatment are produced at points where 

 the division rate of the control is at its maximum. 



