That is the navel, it is dominant on some animals.
Udder.
The female will either have a male attached to her or have an egg sac near her belly. Thanks!
Vacuoles are the little sacs that carry out molecules.
Placenta, is the sac that contain all the blood vessels needed to supplied the unborn during growth in the mother's belly.
when i had my first miscarriage the fetal sac came out in the toilet and it looked like a little veiny blood sac. it was really small almost like a bubble.
gallbladder
Although I am not entirely sure, I think it might be an egg sac or a vocal sac? Not entirely sure though. Try researching it in Google under the description you gave us.
Gallbladder
A cyst is a closed sac containing fluid or semisolid material. An abscess is another type of closed sac containing infectious material.
Egg yolk is the food sac that contains a lot of fats and protein for the embryo and fetus to live off of while it is growing in the egg. The chick literally envelops the yolk sac into its belly to live off of for a few days after it has hatched until it finds food for itself. The yolk is made in the hen, which is developed on the ovary of the hen. The embryo is attached to this yolk sac.
An aerocyst is an air vesicle, a sac or cyst formed under the skin consisting of air.
Normally there is approx 25mL of fluid in the pericardial sac. As little as 100-200mL of rapidly accumulating fluid can cause problems with the normal function of the heart