EFFECTS OF THYROID ON PARAMAECIUM 549 



THE EFFECT OF THE THYROID 



a. On the division rate. In my experiments on Paramaecium 

 aurelia (Race A), a small quantity of thyroid emulsion was 

 added to the hay infusion which is the common laboratory cul- 

 ture medium for this form, and a sharply marked increase in 

 division rate resulted. No such increase was observable in the 

 similarly treated thymus lines. The same method was tried in 

 the first experiments on the caudatum race (B 1) but did not 

 give satisfactory results. Since that time I have conducted my 

 experiments with media prepared by shaking up the gland 

 desiccations in spring water. Many and long continued experi- 

 ments have demonstrated that the thyroid tissue (together with 

 the inevitable bacterial flora) under these conditions presents all 

 the elements necessary^ for the maintenance of life in these in- 

 fusoria. I emphasize this statement in view of my demonstra- 

 tion of the ingestion and digestion of thyroid particles by Para- 

 maecia. Other internally secreting glands either do not sup- 

 ply these elements or form in decomposition (as some of them 

 undoubtedly do) substances lethal to the individuals treated 

 with them (Experiment B 2). 



The thyroid however contains some substance which causes the 

 Paramaecia to divide more rapidly. Experiments have demon- 

 strated that in both species tested the thyroid feeding has pro- 

 duced an increase in the division rate of 65 per cent. In other 

 words a Paramaecium dividing once in twenty-four hours in hay 

 infusion would give rise to 1024 daughter cells in ten days; the 

 same Paramaecium if treated daily with thyroid would produce 

 about 185,000 daughter cells. 



Attempts have been made to identify this substance in iodo- 

 thyrin (table 15). These have failed, perhaps because the prepa- 

 ration (commercial iodothyrin in sugar of milk) was not suffi- 

 ciently active. I have not yet had the opportunity of testing a 

 pure iodothyrin. Other experiments have been conducted with 

 iodine, pure or combined with potassium iodide (table 11). Far 

 from causing any increase in di\dsion rate these preparations 

 proved toxic, causing decreases in the division rate even in the 

 minimal effective dosage. 



