232 CHARLES K. STOCKAKD AND G. N. PAPANICOLAOU 



to the young and lasts for twenty-four hours. Copulation only 

 takes place during heat and if pregnancy fails to occur at the 

 period just after parturition the next heat periods follow after 

 intervals of three to four weeks. 



Konigstein also examined sections of the genital tract giving 

 some important histological descriptions based chiefly on the 

 rat — we shall return to a consideration of these observations in 

 connection with our findings on the guinea-pig. 



Bouin and Ancel ('10), are of the opinion that guinea-pigs do 

 not have a spontaneous ovulation, the process being dependent 

 upon copulation. However, these workers seem to have reached 

 this opinion from observations made on rabbits which were the 

 chief objects of their study. Despite the striking classification 

 which they make of animals having a spontaneous ovulation 

 (monkeys, dogs, horses, cows) and those not having spon- 

 taneous o\ailation (rabbits, guinea-pigs, cats) they admit that 

 rare exceptions are possible and that in any animal an ovulation 

 might occur independently of a copulation. 



C'est la im fait general, mais soumis a des exceptions rares. II 

 peut arriver que des aniinaux a ovulation non spontanee operent la 

 dechirure de leurs follicules murs en I'absence de tout rapprochement 

 sexuel. Nous-meme et M. Villemin avons constate le fait chez le 

 Lapin. M. Miilon vient egalement de I'observer chez le cobaye. 



During the past several years Leo Loeb ('11 a, b) has contrib- 

 uted extensive and valuable studies bearing upon the sexual 

 cycles in guinea-pigs, considering in particular the function and 

 importance of the corpus luteum. Loeb examined a great num- 

 ber of ovaries at different periods, beginning with the time of the 

 first copulation after parturition and concludes, as Rubaschkin 

 1905 and others had previously done, that the cycHc changes in 

 the ovary take place independently of copulation. Loeb thought 

 that the ovulations followed no exact and regular periodicity 

 in all cases. The periodicity differed among the individuals and 

 was influenced by certain external factors, particularly copulation. 

 To quote: 



The exact time at which the new ovulation occurs varies however 

 somewhat in different animals, ovulation occurring earlier in some ani- 



