238 CHARLES R. STOCKARD AND GEORGE N. PAPANICOLAOU. 



For a full description and photographs of the structural changes 

 occurring in the cestrous fluid, the vagina, uterus and ovaries, 

 the reader is referred to the original account. 



After the publication of our results it was found that one of 

 the last papers by Leo Loeb ('14), bearing on a related subject, 

 had unfortunately been overlooked. We regret this, since a 

 discussion of his methods and results would have been somewhat 

 clearer in connection with our full consideration of the oestrus 

 given in the previous paper than in the present connection. In 

 earlier papers Loeb ('n) had completely failed to establish a 

 definite length or periodicity for the sexual cycle in the guinea- 

 pig. In the last paper, however, the length of the cycle was more 

 nearly determined and a very thorough description was given 

 of the microscopic changes taking place in the uterine wall during 

 the heat period. Our independent account of the structural 

 changes in the uterine wall fully confirms Loeb's description. 

 But we are unable to agree exactly with the lengths of the sexual 

 periods as estimated from his examinations of the removed 

 uteri. In a still more recent article Loeb ('18) repeats his 1914 

 estimates and claims the lengths of the sexual cycles to vary 

 between thirteen and a half and nineteen days. 



In all cases Loeb's investigations had centered in a study of 

 the sectioned uterus and ovary, thus necessitating their removal 

 by operation or the death of the animal. Either procedure 

 permits only one observation on a given female. No investiga- 

 tion of the uterus and vagina in the living animal had been 

 made and no continuous observations on the consecutive cycles 

 of given individuals were carried out. 



As mentioned before, we recorded not only the structural 

 changes of the uterus, but almost equally as marked changes in 

 the wall of the vagina. And what we consider to be of still 

 more importance from an analytical or experimental standpoint 

 as a means of estimating the moment of ovulation, was the 

 complete record of the changes in the microscopic composition 

 of the vaginal fluid during the different stages of the sexual cycle. 

 The removal and examination of this fluid is made without in 

 any degree injuring the uterus or vagina and does not interfere 

 with the further use of the female for ovulation and breeding 



