Studies on the ontogeny of the mouse immune system. I. Cell-bound immunity. USNRDL-TR-67-152.

It has been shown that cells which have the potential to differentiate into immunoglobulin-producing cells appear in the yolk sac, liver and caudal half of the embryo by the 9th day of gestation. Late in pregnancy these cells are found in the thymus, gut, lung, spleen, femur and peripheral blood. Certain of the data suggest that immunoglobulin-producing cell lines and those cells which mediate cell-bound immune responses arise early in gestation as separate cell populations. It has been shown that immunoglobulin synthesis per se is independent of the thymus.