EFFECTS OF THYROID ON PARAMAECIUM* 561 



any tendency toward its repetition. I have also considered the 

 question as to whether the results on division rate might not be 

 considered due to more or less unconscious selection. I believe 

 the constancy of the results belies any such objection, but may 

 add that many attempts to increase or decrease the division rate 

 permanently by selection in my thyroid lines have been unsuc- 

 cessful. 



/. General considerations. There seems to be much evidence in 

 favor of the supposition that the thyroid contains a 'dissimila- 

 tory' hormone (Biedl, '13). To this position my results appear 

 to lend themselves. They demonstrate an increased rate of 

 division as an effect of thyroid feeding. Coincident with this a 

 remarkable derangement of the excretory apparatus has been 

 observed. Increase in structure and function are striking evi- 

 dence of an increase in the amount of excretion. Observations 

 on the rate of formations of the gastric vacuoles show that there 

 is no corresponding increase in the amount of food ingested. If 

 without prejudice as to the nature of the chemical reactions 

 taking place we admit the validity of a distinction between 

 assimilatory and dissimilatory phases of metabolism, these 

 facts may be interpreted as an increase in dissimilatory function 

 produced in the metabolism of Paramaecium by thyroid feeding 



SUMMARY AND CONCLUSIONS 



1. Thyroid substance fed to Paramaecium aurelia or cauda- 

 tum, either as emulsion of raw thyroids or as a suspension of the 

 commercial powder, produces a constant and significant increase 

 of 65 per cent in the rate of division over that observed in the 

 common laboratory medium — hay infusion. 



2. The thyroid is the only one of the internally secreting 

 glands that produces this effect. 



3. Boiling the thyroid produces no change in the reaction, 

 lodothyrin and iodine fail to produce the thyroid effect. 



4. Paramaecia after prolonged thyroid treatment revert to the 

 normal division rate when returned to the control medium. 



5. The life history curves of the thyroid treated lines show 

 the same depression periods at the same time intervals as the 



