234 CHARLES R. STOCKARD AND G. N. PAPANICOLAOU 



twenty-fifth day and a half, while fourteen had not yet ovulated. 

 J'ourth, in a lot of six animals killed twenty-six to thirty-four 

 da3^s after the last heat period or copulation, only one had 

 already ovulated. 



A recapitulation of these results may be stated thus: under 

 fifteen days no ovulation; sixteenth day, one; eighteenth day, 

 two; nineteenth day, five; nineteenth to twentieth day, one; 

 over fourteen days and eighteen hours and nineteen days and 

 fifteen hours, thirty-five; twenty-third day, one; twenty-five 

 and a half days, one; over twenty to twenty-six days, fourteen; 

 twenty-sixth to thirty-fourth day, one ; over twenty-six to thirty- 

 four days, five. These figures as Loeb points out do not show 

 any regularity in the occurrence of the ovulation process and, as 

 we shall show beyond, they demonstrate how difficult or almost 

 futile it is to attempt to solve the sexual cycles of an animal by a 

 simple study of the ovarian conditions found on killing the ani- 

 mals at different periods. To anticipate slightly, the figures 

 above show that Loeb entirely failed to discover the presence of 

 a definitely regular periodicity in the ovulation process of the 

 guinea-pig. Thus his examinations though much more thor- 

 ough were as ineffective as those of the previous workers. 



In 1913, Lams gave an instructive review of this problem. He 

 again confirmed the long known fact that a heat period followed 

 parturition in the guinea-pig. The copulation was found to take 

 place within two to four hours after the delivery while ovulation 

 occurred from twelve to seventeen hours after. Thus copula- 

 tion generally preceded ovulation without being its cause. 

 Lams gives no data on the occurrence of later ovulations but 

 devotes himself to a detailed account of fertilization and the 

 early development of the egg. 



A consideration of the sum total of these various observations 

 compels the admission that the opinions concerning the oestrous 

 cycles in the guinea-pig are highly confused and totally unsat- 

 isfactory for application in exact breeding experiments. The 

 one fact which presents itself was estabUshed by the earliest 

 workers and confirmed by all subsequent studies — -that is, that 

 a period of heat follows \vithin the first few hours after parturi- 



