30 VETERINARY OBSTETRICS. 
twenty-four hours only, but appearing again every 
fourteen days until the end of December. 
While these periods may be accepted as the 
average, they are by no means definite, as some 
animals, more especially high-tempered Mares, appear 
always in heat; while others again, the closest examin- 
ation or manipulation fails to detect them in this 
condition at any time, 
In a state of nature and free from artificial 
surroundings, it may be accepted as a rule, that the 
period of cestrum. is so regulated that the young 
animal may be born at a time of the year when its 
maintenance can be most suitably provided for in its 
surroundings, 
If at this period contact with the male is allowed, 
certain changes take place which give rise to impreg- 
nation. The spermatic fluid of the male animal, 
either at or shortly after coition, finds its way to the 
uterus of the female, and from the uterus the 
spermatozoa enter the Fallopian tubes, and pass along 
to the ovarian termination of these structures. 
While it is generally believed that contact with 
the ova of the female takes place in the Fallopian tube 
and at its ovarian extremity, still we have occasionally 
a foetus developed in the ovary, and in this case at 
least, impregnation must have taken place in the ovary 
and not in the tube. 
When impregnation has taken place, wherever it 
may be, the impregnated ovum immediately afterwards 
commences to descend the tube, gains the horn of the 
uterus, becomes attached to the mucous membrane at 
