THE CESTROUS CYCLE IN THE MAMMALIA 49 



litters of pups are seldom if ever produced in a year. Stonehenge l 

 says that there is much individual variability in the periodicity of 

 the cycle, and that " heat " may recur at any interval from four up 

 to eleven months, but that six, five, and four months are the most 

 usual periods. Each bitch as a rule has her own peculiar period to 

 which she remains constant, unless systematically prevented from 

 breeding, in which case the periods tend to recur irregularly or even 

 cease altogether. 2 It has been observed also that the recurrence of 

 the sexual season tends to become irregular with advancing age, and 

 this irrespectively of whether or not the animal is permitted to 

 become pregnant. The periodicity depends also to some extent upon 

 climate, for in Danish Greenland the dogs usually breed only once 

 a year. :i 



The prooestrurn in the bitch is characterised externally by the 

 vulva being swollen and moistened with mucus, and by the existence, 

 usually, but not absolutely invariably, of a flow of blood from the 

 aperture of the vagina. The length of the procestrum is about ten 

 days. The sanguineous discharge generally ceases at the com- 

 mencement of (estrus, which may last for another week or ten 

 days. 



Heape states that the winter oestrus in some breeds does not last 

 so long as the summer oestrus. In certain individuals a relatively 

 slight mucous or sanguineo-mucous flow takes place during the period 

 of (estrus, and may even be continued beyond it, but this is 

 exceptional. Stonehenge states that a bitch will not, as a rule, 

 receive the dog until external bleeding has subsided, and that the 

 most favourable time for successful coition is about the eleventh day 

 of " heat " (in other words, at the beginning of the period of oestrus). 

 This statement is fully borne out by dog-breeders. 



The external changes which occur during " heat" are accompanied 

 by changes in the metabolism, for Potthast, 4 working on the nitrogen 

 metabolism of the bitch, records a slight retention of nitrogen during 

 the "heat" period. A similar result was obtained by Hagemann, 1 

 who states that the retention is followed by a loss of nitrogen after 

 copulation. Murlin" also shows that the effect of the procestrum 

 "is to cause a retention of nitrogen, which may be explained, in part 



1 Stonehenge, Th<> /)<><( in Health J l>;*,:t*e, 4th Edition, London, 1887. 



2 Heapc, /in'. <-it. 



3 Rink, Danish <//wA/,,//, London, 1877. 



4 Potthast, "Kt'imtniss des Kiwrissiimsat/.rs," />/.<.,/,///,,, L-i|i/ig, I VV T. 

 "' Hagemann, " Kiwcissuiasatx. ini tirrisrhcn < trganisnuis," />/.<*'//./ 



Erlangen, 1891. <'f. also S.-hoi udorlt', " Kintlnss der Sdiilddi iis- aiif dm 

 Stoffweehsel," J'tliiifi-Sx .!///., vol. Ixvii., 1897. 



6 Murlin, "The Metabolism of l)c\<-lopmcnt. II. Nitrogm Balance during 

 Pregnancy and Menstruation of tin- I tog," Amer. ./<///. /' /'////.<//., vol. \.\vii., 

 1910. 



