Experiments utilizing chromosome marker and histological techniques in combination with parabiosis and transplanatation procedures have demonstrated an inflow of blood-home stem cells into the chick embryo thymic rudiment. There is close similarity between the histological picture in the chick and in the mouse thymic rudiment, and it is proposed that a similar developmental process takes place in both. It may be concluded that the epithelial component of the thymic rudiment, rather than producing lymphoid cells itself, furnishes an inductive environment for the proliferation and differentiation of stem cells derived extrinsically.

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