ABSTRACT
Following the development of the Immune System from the inferior Vertebrata to the Mammalia and Man, we can see a progressive and gradual improvement of the cell-mediated immunity and humoral immunity and a progressive anatomic-functional organization of the primary lymphoid organ responsible for the maturation of T lymphocytes (Thymus); we can find a primary lymphoid organ of the humoral immunity only in Birds (Bursa of Fabricius = B.F. , or cloacal Thymus).

The lack of a bursa-equivalent organ in Mammalia, in which the humoral immunity reaches the maximum improvement, seems to be unjustified. Comparing the embryonal development and the B.F. anatomy with the embryonal development and the definitive arrangement of the human caudal region, you can note that the ontogenesis of the vascular structures of those regions is practically the same in both species. In chicken, the B.F. is bedewed by a branch of the Medium Sacral Artery (MSA); in man the MSA ends in the coccygeal gland. Bursopoietina (a peptide which stimulates the appearance of antigens of differentiation in B Lymphocytes) has been isolated from B.F. taken from the chicken after the birth (not from embryonal and immature BF). The production of this "filterable factor" assigns the BF the role of endrocrine gland. The most important endocrine glands derive from extroversions of the primitive intestine; at first they are connected to it; then by evolving they become anatomically and functionally autonomous.

BF ontogenesis let us predict that in Mammalia the bursa-equivalent organ will undergo the typical trasformations of the endrocrine glands annexed to the Primitive Intestine. The Coccygeal gland has an histological structure very similar to Thymus and BF: Testut says: "it is probably an endocrine gland, but its function is unknown". The comparison between the BF embryonal development and the one of the caudal regions in chicken and in man; the comparison between Thymus, BF and Coccygeal gland anatomy; the transformations that, during Evolution, the endocrine glands annexed to the Primitive intestine undergo, that are analogous to the transformations that you can find also in the BF development; suggest that:

• whereas in chicken the production of maturation factors and the maturation of itself B lymphocytes occur in only one anatomic site;
• in man the evolution brings to a clear-cut separation between functions and sites: the maturation factors of B lymphocytes are secreted by a specific endocrine gland, the Coccygeal gland (during the evolution it has lost the connections with the Primitive Intestine that are still present in chicken, an ferior Vertebrate);
• the site where B lymphocytes mature (protected also by interferences of external antigenic stimuli) is at first in the fetal haematopoietic organs, secondly in the bone marrow.