THE (ESTRUS-PRODUCIXG HORMONE 95 



J'agiiial reaction. The discovery by Stockard and Papani- 

 colaou that the vaginal changes in the rodent could be used to 

 determine the oestrous cycle in the intact animal has made 

 possible a very great advance in the technique of testing for 

 oestrus-producing activity. This technique now consists of in- 

 jection of the ovariectomized test animal, and examination for 

 the appearance of the characteristic cornified cells in the 

 vaginal contents. The reaction time is fairly regular. 

 Allen and his co-workers (24) reported that the initial changes 

 in the vaginal smear of the ovariectomized animal could be 

 detected 40-48 hours after injection, but active growth in the 

 vagina and uterus precede these changes. Many workers have 

 since shown that the latent period for the change in the vaginal 

 smear is about two da^'s. The reaction time is not affected by 

 the size of the dose — 20 units produce no quicker effect than 2 

 units (82, 138). Also, the age of the animal does not affect the 

 time, since immature animals show the characteristic changes 

 as soon as adults. 



The methods of collecting the vaginal contents vary in detail, 

 and individual workers have their own particular methods. A 

 more serious lack of uniformity in the vaginal smear technique 

 is that while most workers consider that the appearance of full 

 oestrous symptoms, including cornification, is essential for a 

 positive reaction, others, including Laqueur and his collaborators 

 (345) and Lipschiitz (372) , have used the prooestrous smear as in- 

 dicating a positive result. Loewe and his co-w^orkers (416) make 

 a cell-type count on the vaginal contents and analyse the results 

 for cestrous periodicity. This method may result in an early 

 prooestrous smear being considered as a positive result, and in 

 any case it is unnecessarily cumbersome. In spite of the variety 

 of intermediate smears which can be obtained, as pointed out by 

 Brouha and Simonnet (96, 97), the vaginal smear technique has 

 a comparatively clear end-point, and the real difficulty in the 

 assay of oestrus-producing extracts is not to determine w^hether 

 or not oestrus has been produced in any one animal, but to 

 arrive at some generaJly applicable basis for dealing with the 

 variation found in individual response to a given amount 

 of the hormone. The use of the prooestrous smear as a 

 positive reaction can be supported on the grounds that a small 



