2004). B-lymphocytes, the cells that produce antibodies, are initially produced in the embryonic liver, yolk sac and bone marrow, then move through the blood to the bursa of Fabricius (BF). Birds have a bursa of Fabricius (BF), which is an outpocketing of cloaca.
Most squirrels are vegetarians. There are a few exceptions like the flying squirrel. The flying squirrel will eat small birds, insects and definitely steal eggs from birds nest in order to make a meal. Generally speaking though, squirrels will mainly stick to things that are vegetarian related.
Squirrels are wild animals and this affects what they should eat in captivity. Squirrels can subsist on pre-made food designed for rodents, but they should also get fresh foods to round out their diet. It's also important to make sure your pet is getting the right balance of nutrients essential to squirrel health.