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The histogenesis of the mouse thymus was studied by means of light and electron microscopy in an attempt to clarify the simultaneous development of stromal and lymphoid cell populations. On the twelfth day of embryonic life, the thymus primordium was composed principally of undifferentiated epithelial cells and some lymphoblasts. In the developing cortical regions, lymphoblasts accumulated rapidly, stretching the epithelial cells which became stellate in shape. The latter contained multivesicular bodies and, from the sixteenth day on, also typical clear vacuoles. Medullary regions were prefigurated as soon as day 13 by several areas wherein lymphoblasts were sparse and epithelial cells were closely associated, with numerous desmosomes and abundant tonofilaments. On the sixteenth day, some epithelial cells in these regions were differentiated into globular cells, or formed Hassall's corpuscles and intra- or extracellular cysts. On the seventeenth day, the presence of interdigitating cells in the medullary areas completed cortico-medullary differentiation. On the eighteenth day, small cortical thymocytes differentiated and the thymus possessed all characteristics of an adult thymus. Thus, at birth, the histogenesis of the mouse thymus was achieved and the only further modification consisted in a gain of weight.