102 INTERNAL SECRETIONS OF THE OVARY 



that 2-5 mgms. of an extract would produce cestrus in about 

 10^0 of ovariectomized rats, while 17-5 mgms. would pro- 

 duce cestrus in only a little over 80%. The interpolation of 

 intermediate values gave the usual S-shaped curve of the type 

 found by Trevan (614) to be typical of the response of groups of 

 animals to various drugs and poisons. As a result of this work, 

 Coward and Burn suggest that the unit should be defined as the 

 amount necessary to bring 50% of a group of ovariectomized 

 animals into oestrus. Laqueur (345), however, requires a 75^0 

 response for a unit of activity. In practice, this technique can 

 be applied fairly easily, once the standard curve for the strain of 

 animals in question has been worked out. Thus, if a trial 

 injection brings into oestrus 40% of a group, the amount 

 required to bring 50% can be arrived at from the standard 

 curve. Even such methods of dealing with the problem hardly 

 make assay quite satisfactory. The size of group suggested by 

 Coward and Burn is 20 animals, but Dodds and co-workers 

 (5) found that using this comparatively small number diffe- 

 rent groups may show quite a different percentage response 

 to the injection of a given amount. Further, a group may show 

 a different percentage response at different times, and it seems 

 difficult to correlate this variation with the time after ovariec- 

 tomy, or with any other variable. It would seem, therefore, 

 that, unless prohibitive numbers of animals are used in each 

 group, the assay of oestrin cannot become exact with the pre- 

 sent methods. 



Number of injections. Working with the old fat -soluble 

 preparations, various authors found that a dose given in a series 

 of small injections produced no greater effect than the same dose 

 given as one injection, and, as a rule, therefore, the fat -soluble 

 extracts were administered as one injection. With the 

 recent water-soluble preparations and with the purer fat 

 extracts, however, it has been found that a given amount will 

 produce much less effect when given as one injection than when 

 given as a series of injections at short intervals. This is pre- 

 sumably due to the rapid absorption, and destruction of the 

 hormone in the blood when administered in a comparatively 

 pure form (5, 438). 



Absorption. With the cruder fat extracts, the reaction to one 



