310 WALDO SHUMWAY 



vitality, but the thyroid treatment resulted in the greatest in- 

 crease in division rate observed at any time. 



The explanation, I believe, lies in the fact that in these two 

 Experiments the Paramaecia were at very different stages in their 

 life history and that their protoplasmic make-ups were not com- 

 parable. I believe that in Experiment V a typical depression 

 period (Calkins) or rhythm (Woodruff) was occurring, from which 

 the race was able to recover without stimulation, while the 

 thyroid was able to produce its usual effect. In Experiment IX 

 however I suggest that we have a period of depression leading to 

 "germinal death" such as carried off Calkins' race of Para- 

 maecium caudatum. Under this hypothesis we can understand 

 how the thyroid was without effect in the final depression 

 period, for if some element of the protoplasm necessary for 

 digestion were lacking, the thyroid agent could not be released 

 or act. While this is advanced only as an hypothesis, it is a 

 significant fact that at the time when the race was nearing the 

 end of its life cycle, the thyroid which had previously produced a 

 significant increase in the rate of division with such absolute 

 uniformity, presented no perceptible effect. Attention is also 

 called to the fact that this race of aurelia died out in four hun- 

 dred and twenty generations, while Woodruff's race of the same 

 species has been maintained for more than four thousand gen- 

 erations, one more example of the great differences in vitality 

 manifested by different races of Paramaecia. 



SUMMARY 



Experiments have been carried on to demonstrate the effect 

 of thyroid and thymus added to the diet of a pedigreed line of 

 Paramaecium aurelia during a life history of four hundred and 

 twenty generations. These experiments have shown that the 

 effect of the thyroid is to increase greatly the rate of division, 

 except at the time when the line was nearing the close of its cycle; 

 and further that the efTect is not permanent after feeding with 

 thyroid is stopped. Similar treatment with thymus gave nega- 

 tive results. It has been shown further that the Paramaecia 

 actually ingest and probablj'' digest particles of thyroid. 



