This lymphoid organ is peculiar to birds. It is found in the proctodeum. It is easily distinguished from the thymus if you note the absence of thymic corpuscles, the fact that each cortex and medulla appears to be separate and also the presence of a pseudostratified columnar epithelium lining the bursa. Recall that B lymphocytes develop in an antigen-free environment in the bursa. Bursa-equivalent lymphoid tissue has not been identified in mammals.
