THE (ESTRUS-PRODUCING HORMONE 103 



large injection would be spread over several days (505), 

 owing presumably to delay in absorption. The effect of 

 purer fat extracts, on the other hand, was less prolonged, 

 probably owing to more rapid absorption. \Mth the present 

 increase in the purity of extracts this problem becomes of 

 importance. It has been found (438) that (a) maximum 

 activity is obtained when the administration of a dose is 

 spread over 24 to 36 hours, with intervals of not more than 

 two hours between injections, and (b) within limits the reaction 

 is not influenced by the volume of the injections. 



Time of testing after operation. Following ovariectomy, both 

 the uterus and the vagina begin to undergo atrophy, and in time 

 these organs become entirely degenerate. It seems probable 

 that one result of this degeneration would be an entire lack of 

 sensitivity to ovarian activity, and it is necessary to consider, 

 therefore, how far this degeneration can go without invalidating 

 any tests done on such animals. In other words, it is probable 

 that the longer the injection is postponed after ovariectomy the 

 greater would be the amount of hormone required to produce 

 positive effects. Injection at regular intervals might, on 

 the other hand, prevent the atrophy of the organs and 

 maintain sensitivity. No precise experimental work appears to 

 have been done on this problem. Allen and his collaborators 

 (24) report that the duration of the operative anoestrus has 

 very little effect upon the amount subsequently required to 

 produce oestrus. These authors found no decrease in sensi- 

 tivity up to fifteen weeks after the operation. Kahnt and 

 Doisy (318), hov/ever, ' prime ' the animals before carrying out 

 a test. 



WeigJit of the animal. Following ovariectomy, the female 

 rodent very soon deposits large amounts of fat and greatly 

 increases in body weight. The influence of this increased 

 weight on the amount of oestrin required for a positive reaction 

 has been studied by Bugbee and Simond (no), whose results 

 are summed up in Table 5 (p. 104). 



These results show that as the animal gets older and heavier 

 more oestrin is required for a positive reaction, and the assayed 

 strength of a preparation is less. Correcting for the weight of 

 the animal gives a more constant result, and Bugbee and 



