GENETIC TYPE AND THE ENDOCRINES 693 



tract was again administered in doses of 2 gms. daily. Within 

 about 20 days a definite effect on the reaction could be ob- 

 served, and this effect thereafter increased enormously. The 

 magnitude of the maximal response was more than four times 

 that of the maximal response of the preceding thyroid de- 

 ficient period. The response increased still further for a time 

 after the thyroid was withheld and was strong for 29 days 

 afterward. A weak reaction was elicitable even after this 

 time but disappeared entirely within about 20 days more. 

 As the conditioned reflex improved on each occasion during 

 the thyroid treatment, the average latent period of the re- 

 sponse was shortened. The latent period was decreased in 

 length from 8 seconds in the deficient intervals to 5 seconds 

 in the treatment intervals. This fact illustrates in a quanti- 

 tative way the great increase in alertness brought about by 

 the administration of the extract. 



In the last stage of the experiment, when no extract was 

 being administered, the study of the salivary conditioned 

 reflex was discontinued. With the animal in a low condition, 

 attempts were made to establish the conditioned motor reflex 

 to a tone of 256 cycles. The attempts were only partially 

 successful. A conditioned response did not appear until the 

 fourteenth trial, when a slight twitch of the reaction leg- 

 muscles was seen. The paw was not lifted from the floor. 

 This rate of formation of the reflex is exceedingly slow. 

 From this point on, the slight twitch of the limb was observed 

 only twice in twenty-five consecutive trials. The shock always 

 evoked an equally slight leg tremor. 



We may conclude from the above cases that the efficiency of 

 the positive C-R (both salivary and motor) decreases after 

 thyroidectomy. Simultaneously, the negative C-R increases. 

 To interpret the increased efficiency of the negative as " dis- 

 crimination' ' or "differentiation" on the part of the thyroid- 

 less animal would be an absurdity which is entirely inconsistent 

 with observation on cretinous individuals. It is also incon- 

 sistent with our own observations on the behavior of operated 

 animals under positive stimulation. The latent period is 



