In the case of the fowl—(1) Fertile eggs can be expected within 24–48 hours after the introduction of the male, though the onset of fertility varies with different matings.(2) The length of life of the sperm within the body of the female is about 15–20 days, though in exceptional cases it may be greater.(3) Though fertility endures 15–20 days after the removal of the male, the eggs laid after the first week commonly fail to complete their development.(4) If after the removal of the male a second male is introduced, the influence of the first sire is removed by the 7th to 10th day, the exact time differing in different cases. There is a relation between the general “vigour” of the individual and the fertilising power of the sperm it elaborates.