120 CYCLIC CHANGES IN THE OVAKIES AND UTERUS OF THE SOW, ETC. 



with a simple, diffuse placenta. In short, the problem is here reduced to its lowest 

 terms, and the solution promises, therefore, to be the more useful as a contribution 

 to the general theory of the reproductive cycle. 



EXTERNAL MANIFESTATIONS OF THE REPRODUCTIVE CYCLE IN SWINE. 



Investigations in the physiological anatomy of the reproductive system must 

 depend upon material collected at accurately determined periods of the ovarian- 

 uterine cycle. For this reason it will be necessary to review briefly the well-known 

 facts of the sexual manifestations of swine. 



Sexual maturity is attained before the age of one year, sometimes as early as 4 

 months, often before the uterus has attained its full adult dimensions. Maturity 

 is characterized by the recurrence of periods of sexual activity ("heat," or oestrus) 

 at intervals of 2 to 4 weeks, the usual interval being 21 days. An exact study of 

 the duration of the oestrous cycle has been made by Struve (1911), whose results 

 agree closely with the figures just given; his computations give a mean interval of 

 20. 66 0.205 days, with a standard deviation of 2.36. The curve of frequency 

 distribution of observed cases shows the shortest interval to be 15 days, the longest 

 30 days, but 75 per cent of the animals fall within the limits of 18 to 23 days. The 

 duration of oestrus is commonly 3 days, during which time the rutting sow exhibits 

 excitement in the presence of the male, with ready acceptance of coitus. If no male 

 is present the sow will follow the other females about the pen, sniffing at their 

 genitals and frequently going through an imitation of the sexual act by "riding" 

 upon the others; or she may at times be the recipient rather than the donor of these 

 attentions. In a large herd without boars the females in heat will often be found in 

 a separate group apart from the others, where for hours at a time the exhibition of 

 the cestral urge continues, interrupted only by siesta or feeding time, until at the end 

 of 3 days, or occasionally longer, it subsides, to be followed by an interval during 

 which sexual activities are in abeyance. 



At the height of oestrus the vulva is often slightly everted, swollen, and 

 reddened, and there is sometimes a slight serous or mucous vaginal discharge. 

 Occasionally the discharged fluid is flecked or stained with blood, but from internal 

 examination of the reproductive tract it is apparent that in these cases the bleeding 

 is of external origin, 'caused no doubt by trauma to the vulva; in this detail, as in 

 all others of the description just given, the external manifestations of the sexual 

 cycle are in complete contrast to those displayed by the human and other primate 

 races. 



The 21-day cycle appears to continue regularly throughout the year unless 

 interrupted by pregnancy. In pregnant animals received at the packing-houses, 

 fetuses are found in all stages of development without much regard to the time of 

 year. Pregnancy can begin only at an cestral period, since then alone is copulation 

 permitted. The span of gestation is 16 to 17 weeks, usually 116 to 120 days. As 

 with other mammals, the cestral periods do not occur during pregnancy, but 

 according to Struve (1911), oestrus recurs about 4 to 9 days after parturition. - 



