Differences in the structure of the interphase nucleus between two species of birds, the Japanese quail (Coturnix coturnix japonica) and the chick (Gallus gallus) have been used to distinguish cells from different origins in interspecies combinations. This biological cell marking technique was applied to thymus and bursa of Fabricius histogenesis. Using various combinations between components of quail and chick thymic and bursic rudiments, the respective contribution of endodermal epithelium, mesenchyme and blood-borne extrinsic elements to the histogenesis of thymus and bursa was analyzed. It was demonstrated that the whole lymphoid population in these organs is derived from immigrant blood-borne stem cells. Thymus and bursa become attractive for stem cells at a precise stage of their development. In thymus the attractive period last about 24 hours in the quail and 36 hours in the chick. In the bursa of Fabricius the invasion lasts several days in both quail and chick embryos. The mechanisms which control the onset and the interruption of the stem cells inflow have been investigated.