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During the prenatal period. the testicles in a male puppy develop inside the abdomen. As part of the maturing process, at or near birth, the testicles descend through an opening in the muscular abdominal wall in the groin. This opening is called the inguinal canal. Next, the immature testicles move further into the scrotum, which is their adult position.
Since males are barred from the show ring without two testicles in the scrotum, breeders and their veterinarians begin checking male puppies early. In the majority of male puppies, the gonads can be felt in the scrotum as young as 6 weeks. Often, even if their presence cannot be found in the scrotum, they can be palpated up in the flank near the inguinal ring.
Occasionally, one or both testicles are truly undescended, remaining inside the muscular abdominal wall. This condition is called cryptorchidism (Latin for "hidden testicle''), unilateral for one or bilateral for both. The slang term monorchid is often used to refer to a unilateral cryptorchid .
Testicles that are not out of the abdomen by to 5 months of age probably won't appear. Often, if the testicles were palpable at 5 months, the hidden testicle may be high up in the flank and not completely inside the abdomen. In rare cases, a descended testicle retracts into the abdomen.
Cryptorchidism is not a dangerous condition. The retained testicle cannot develop sperm, due to the high temperatures of the interior of the body. It can, however, produce testosterone, which is not temperature dependent. Thus, a bilateral cryptorchid is sterile but still virile. Retained testicles are about 10 times as likely to develop testicular tumors as properly descended ones, although this does not mean that all undescended testicles will become cancerous.
Proper descent of testicles is an inherited condition, and all AKC breed standards disqualify cryptorchids. Thus, a cryptorchid dog is never a breeding animal. and dogs with undescended or ''high flank'' testicles should be neutered. This removes all chance of testicular tumors.
Author(s): Wilcox, Bonnie, D.V.M.
Canine Library: Reproduction