Cryptorchidism
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This was provided by Golden Animal Hospital.
During growth of a male fetus, the testicles develop in the abdominal cavity, pass through an opening in the body wall called the inguinal canal, and descend into the scrotum. Normally both testicles have descended into the scrotum at or shortly after birth. Occasionally descent is not complete until 5-6 months of age.

In some individuals, however, one or both testicles fail to descent into the scrotum. Dogs with both testicles undescended are usually sterile while those with only 1 testicle undescended are fertile. Undescended testicles are most common in small or toy breeds.