ASSAY OF CORTICAL HORMONE 257 



Since the survival of animals is determined to some extent 

 by the diet used, this should be standardized and remain un- 

 varied throughout the period of assay. Rats, even in adrenal 

 insufficiency, may be tempted by choice morsels which will 

 give the false impression of growth. Since only a short time is 

 necessary for the completion of the assay, the addition of 

 lettuce to the above described diet twice weekly, as ordinarily 

 done in protracted experiments, is unnecessary. 



In assaying a given preparation, a measured amount is 

 injected or preferably mixed with the animal's food. The 

 author has found that the use of six rats for each test precludes 

 the vitiating factor due to individual variations, if one has 

 available a stock of animals of uniform quality. When the 

 hormone is orally administered one obviously must make allow- 

 ance for the daily food requirement of the animals. On the 

 day following operation, each of the animals consumes about 

 5 grams of the mixture described above. During the course of 

 a week of treatment their food intake gradually increases to 

 about 10 grams per rat per day. With practice one easily 

 estimates the quantity of food necessary for a day and this 

 amount is mixed with a given dose of the hormone daily. The 

 rats are weighed daily at the same hour. It may be objected 

 that with oral therapy as just described each rat does not 

 necessarily ingest an equal share of the hormone nor is the 

 daily allotment always exactly equal to the daily food require- 

 ment. As a matter of fact, these objections are not of great 

 practical significance. Any slight over- or under-estimate of 

 the daily food requirement is compensated for by the next 

 day's feeding. Individual variations in the food intake of the 

 different animals can be avoided by maintaining each rat in a 

 separate cage. 



In Figure 15 are reproduced typical curves showing the 

 average gain or loss in weight of groups of six rats maintained 

 under various conditions. The lower curve demonstrates the 



