306 WALDO SHUMWAY 



DISCUSSION 



In the experiments described above, we have seen that in 

 every case, disregarding for the moment Experiment IX, the 

 thyroid treated Une has divided more rapidly than the control 

 line from which it was descended. Thymus treated lines on the 

 contrary, while apparently showing a decreased rate of division, 

 have in reality given only negative results, for the lower rate of 

 division was caused by a greater number of deaths. Lines 

 returned to the control medium after several periods of thyroid 

 treatment have then given the same average division rate as the 

 control, while their sister lines under continuous thyroid treat- 

 ment have continued at the higher rate of division. 



The source of the glands used bears no relation to the effects 

 produced. A typical thyroid increase was produced by prepa- 

 rations from fresh glands of rats, rabbits and cattle, and from 

 dried glands of sheep. The stability of the agent is shown by the 

 fact that emulsiors have continued active for twenty-five days 

 durirg which time putrefactive decomposition had commenced. 

 The accompanyirg graph (diagram 3) shows the effect of two 

 emulsions (Thyroid II and III) the one from rats, the other 

 from rabbits, for five successive periods. It will be seen that the 

 two curves are similar, although at this time I have not sufficient 

 data to discuss the nature of the curve. 



I was however unable to secure any positive results with fil- 

 tered extracts, either from fresh or dried material. This is the 

 reverse of the results of Nowikoff ('08) who has pubhshed the 

 only other experiments on this question I have been able to dis- 

 cover in the literature. It might be well then to discuss this 

 paper in some detail. 



While studying the chemotaxis of Paramaecium to thyroid 

 extract, Nowikoff observed that the individuals treated with 

 thyroid seemed to divide more rapidly than those kept in hay 

 infusion. He devised some experiments intended to dem- 

 onstrate this increase in division rate: from these he concluded 

 that the thyroid extract "exerts an intensive influence." Nowi- 

 koff used Merck's Glandula Thyroidea sice. pulv. dissolved in 



