38 THE PHYSIOLOGY OF REPRODUCTION 



only is the usual duration of the period during which dicestrous 

 cycles recur in the domestic rabbit, and that if oestrus is experienced 

 in winter it may occur independently of the possibility of pregnancy, 



The duration of the dioestrous cycle varies considerably. "While 

 some individuals exhibit oestrus every three weeks fairly regularly, 

 others do so every ten days ; on the whole, I think ten to fifteen 

 days is the usual length of their dioestrous cycle." 1 In Lcj>t'* 

 variabilis recurrent dioestrous cycles are probably continued for 

 about two months. 2 



The squirrel (Sciurus vulgaris) in Britain, according to Heape, 

 is probably moncestrous ; but this animal, in Southern Europe and 

 Algiers, according to Lataste, is apparently polycestrous. In Britain 

 squirrels breed early in the year, and sometimes have a second litter 

 in August. 



It is difficult to determine the length of the procestrum and 

 oestrus in Rodents, since the external changes which characterise 

 these conditions are comparatively slight. Heape says that the 

 procestruiu in the rabbit lasts, probably, from one to four days. At 

 this time the vulva tends to become swollen and purple in colour, 

 but there is no external bleeding. According to Long and Evans 3 

 the cycle is marked by characteristic changes in the cellular and 

 fluid content of the vagina. Lataste 4 states that external bleeding 

 occurs during the " heat " periods of Pachyuromys duprasi, Dipodillus 

 simoni, and Meriones shawi. 



The guinea-pig (Cavia porcellus) in captivity can become pregnant 

 at any season, but more frequently in the summer than in the winter. 

 Stockard and Papanicolaou 5 state that oestrus occurs every sixteen 

 days and that the vagina just before these periods becomes filled 

 with mucus. During the dioestrum there is very little fluid to be 

 found in the vagina. 



Loeb 6 had previously found that the dioestrous cycle lasted from 

 twenty to twenty-five days, or in animals prevented from copulating, 

 from fifteen to nineteen days. " Heat " rapidly succeeds parturition, 

 as in the case of so many other Rodents. 7 The period of gestation is 



1 Heape, loc. cit. 



2 The dioestrous cycles may be interrupted by a period of pseudo-pregnancy 

 initiated by a sterile coition. See below, p. 101. 



3 Long and Evans, loc. cit. 



4 Lataste, loc. cit. 



\ Stockard and Papanicolaou, " The Existence of a Typical (Estrous Cycle in 

 the Guinea-Pig," Amer. Jour. Anat., vol. xxii., 1917. 



6 Loeb, "The Cycle Changes in the Ovary of the Guinea-Pig," Jour, of 

 Morph., vol. xxii., 1911. "The Correlation between the Cyclic Changes in the 

 Uterus and the Ovaries," Biol. Bull., vol. xxvii., 1914. 



7 Sobotta, " tiber die Bildung des Corpus Luteum beim Meerechweinchen," 

 Anat. ffefte, vol. xxxii., 1906. 



