Maachalos Assuros 7:13

The cords of the heart, the front leg, the spinal cord, the lower jaw, on either side of the tongue, those in the fat of the small intestine that are like spider webs, the membrane over the brain and the membrane on the testes are all prohibited because of blood.

Maachalos Assuros 7:14

If a kid or a lamb is younger than 30 days old, one may cook its testes without peeling them; after 30 days, if thin red lines are visible, then it’s clear that blood has circulated through them and one may not cook them without first removing the membrane or cutting them open and salting them, as has already been discussed. If thin red lines have not yet appeared, they remain permitted.