Detailed cytological studies of serial sections have been performed on 85 golden hamster embryos ranging between 9 and 16 days (4–27 mm) of embryonic development.

Sequential cytological changes in the development of the thymus of the hamster into an active lymphocytopoietic organ indicate that lymphoblasts develop by the gradual proliferation and transformation of “undifferentiated” epithelial cells comprising the primordial thymus. The “undifferentiated” epithelial cells undergo two distinct lines of differentiation during early development of the thymus: into lymphoblasts and into stellate reticular-epithelial cells which form the organ parenchyma. Transformation, both of lymphoblasts and reticular-epithelial cells begins during the eleventh day of development prior to the separation of the thymus and parathyroid.

That the lymphoblastic and lymphocytic elements appearing in the embryonic thymus of the hamster are of epithelial rather than mesenchymal derivation is indicated by the following: (1) the absence of lymphocytes, lymphoblasts and hemopoietic activity in the connective tissue surrounding the embryonic thymus before and during the period of initial lymphoblastic formation; (2) the presence of a continuous basement membrane surrounding the developing thymus; (3) absence of cells passing through the basement membrane during this phase of development; (4) the absence of vascularity or vascular invasion of the thymus until after the appearance of lymphoblasts in the thymic parenchyma; (5) the demonstration of a sequential series of morphological transitions between “undifferentiated” epithelial cells and lymphoblasts and (6) the subsequent homoplastic proliferation and maturation of the lymphocytic elements from lymphoblasts in the developing thymus.