Cryptorchidism is a condition where one or both of the testicles are not in their correct position. Instead of being in the scrotum, they are either in the inguinal canal (where the leg meets the belly) or inside the abdomen. It is much more common to happen in dogs, but as I recently was exposed to (2 cats in 2 months) it can also occur in cats as well. I should first explain how this happens and then will discuss what should be done about it and why.
The testicles develop in the abdomen near the kidneys. In the late stages of the fetus and then after a puppy or kitten is born the testicles make a slow descent from the abdomen, through the inguinal canal (small opening between muscle layers of the abdomen and leg), and eventually into the scrotum where they will live. The testicle is essentially attached to the scrotum by a ligamentous structure called the gubernaculum. This structure responds to certian hormones during the developmental life cycle and pulls the testicle to its normal position. Typically the testes should be out of the inguinal canal at birth and then by six months should have made their way to the scrotum.
In the case of the neuter, the testicle may be located any place along the descent of the testicle. In many cases the testicle can be palpated in the inguinal canal and a small incision over that area is all that is need to perform the neuter. In other cases where the testicle is not easily found in the inguinal canal, the veterinarian may need to perform an abdominal exploratory to locate the testicle, as it could be any place from near the kidney to right by the inguinal ring.
If your pet only has one or no descended testicles, don't panic. We typically give them until 6 months of age to drop and then if they haven't we can perform a specialized neuter to remove them.