226 CHARLES K. STUCKAKU AND G. N. PAPANICOLAOU 



ovulation times aiitl heat seasons. The reason for such a lack 

 of knowledge is that these small rodents do not reveal in a very 

 evident manner the existence of their typical sexual rhythm as 

 do many mammals of other classes. 



The guinea-pig never, or only in rare cases, shows an external 

 flow from the vagina, and there is no easily noticeable change in 

 the appearance of the external genital organs during the differ- 

 ent periods of sexual activity. The only expression generally 

 obserA'ed of the sexual condition or heat jDeriod in the female 

 is her willingness to accept the male, and this sign is, of course, 

 only manifested when a male is present and a copulation takes 

 place. The copulation then brings about the disturbing factor 

 of pregnancy and the observation of the return of the heat 

 period is prevented. The practical difficulties in observing 

 successful copulation in these animals makes the study of their 

 sexual conditions still more difficult. 



Marshall ('10), in a recent smnmary has stated the case as 

 follows : 



It is difficult to detorniine the length of the prooestrum and (wstnis 

 in rodents, since the external changes which characterize these condi- 

 tions are comparatively slight. Heape says that the prooestrum in 

 the rabbit lasts, probably, from one to four days. At this time the 

 vulva tends to become swollen and purple in color, but there is no 

 external bleeding. The same may be said of the rat and the guinea- 

 pig; but, in the experience of the writer, it is generally impossible to 

 detect the prooestrous condition in either of these animals with aljsolute 

 certainty. 



It must be recalled here that Marshall has devoted a great 

 deal of study to this subject. 



The difficulty in observing signs of heat in the guinea-pig has 

 led a numbers of workers during the past fifty years to a study of 

 the ovaries in order to establish the ovulation cycle. The re- 

 sults of such studies, as we shall point out beyond, are inaccurate 

 and confusing in all cases. 



Recognizing the above state of affairs, w^e determined to as- 

 certain whether by a more minute examination of the genital 

 organs of the female it might not be possible to observe an oestrous 

 cycle. In order to examine the vagina thoroughly we have in- 



