The present paper, by using interspecific chimera of chick and quail and submicroscopic study on the bursal follicle associated epithelium (FAE) cells during differentiation, indicates that FAE cells of the bursa of Fabricius are formed not by differentiation of the epithelial cells but originate from the mesenchymal cells after invading the bursal anlage, and then differentiate into FAE cells. A new suggestion is given on the interpretation of the mechanism of the invasion of hemopoietic stem cells into the bursal epithelium.