In the stallion sterility may be due to the following causes: (a) Imperfect development of the testicles, as in cases in which they are retained within the abdomen; (b) inflammation of the testicles, resulting in induration; (c) fatty degeneration of the testicles, in stallions liberally fed on starchy feed and not sufficiently exercised; (d) fatty degeneration of the excretory ducts of the testicles (_vasa deferentia_); (e) inflammation or ulceration of these ducts; (f) inflammation or ulceration of the mucous membrane covering the penis; (g) injuries to the penis from blows (often causing paralysis); (h) warty growths on the end of the penis; (i) tumors of other kinds (largely pigmentary), affecting the testicles or penis; (j) nervous diseases which abolish the sexual appetite or that control the muscles which are essential to the act of coition; (k) azoturia with resulting weakness or paralysis of the muscles of the loins or the front of the thigh (above the stifle); (l) ossification (_anchylosis_) of the joints of the back or loins, which render the animal unable to rear or mount; (m) spavins, ringbones, or other painful affections of the hind limbs, the pain of which in mounting causes the animal to suddenly stop short in the act. In the first three of these only (a, b, and c) is there real sterility in the sense of the nondevelopment or imperfect development of the male vivifying element (spermatozoa). In the other examples the secretion may be imperfect in kind and amount, but as copulation is prevented it can not reach and impregnate the ovum. In the mare barrenness is equally due to a variety of causes. In a number of breeding studs the proportion of sterile mares has varied from 20 to 40 per cent. It may be due to: (a) Imperfect development of the ovary and nonmaturation of ova; (b) cystic or other tumors of the ovary; (c) fatty degeneration of the ovary in very obese, pampered mares; (d) fatty degeneration of the excretory tubes of the ovaries (Fallopian tubes); (e) catarrh of the womb, with mucopurulent discharge; (f) irritable condition of the womb, with profuse secretion, straining, and ejection of the semen; (g) nervous irritability, leading to the same expulsion of the male element; (h) high condition (plethora), with profuse secretion and excitement; (i) low condition, with imperfect maturation of the ova and lack of sexual desire; (j) poor feeding, overwork, and chronic debilitating diseases, as leading to the condition just named; (k) closure of the neck of the womb, temporarily by spasm or permanently by inflammation and induration; (l) closure of the entrance to the vagina through imperforate hymen, a rare, though not unknown, condition in the mare; (m) acquired indisposition to breed, seen in old, hard-worked mares which are first put to the stallion when aged; (n) change of climate has repeatedly been followed by barrenness; (o) hybridity, which in male and female alike usually entails sterility.