Cell division of thymus lymphoid cells from 30- to 60-day-old embryonic guinea-pigs, as well as young adults was investigated on cell smears stained with Giemsa. Asymmetrically dividing cells were found in the developmental stage of thymocytes. At the telophase of such cells, the cytoplasma of one of two immature daughter cells was apparently larger in amount than that of the counterpart cell. When 40-day embryonic lymphoid cells were examined, most of the asymmetrically dividing cells at telophase belonged to the larger cell population (mean diameter 15.7 micrometer). The mean diameters of the larger and smaller immature daughter cells of these dividing cells were estimated as 12.0 and 9.2 micrometer, respectively. Ultrastructural study also revealed a lymphoid cell which was under-going in situ apparent symmetric division at telophase. One of two immature daughter cells alone possessed a large cytoplasmic protrusion with few organella. The frequence of asymmetric division calculated by a rough estimation was around 10% of the total cell division between days 30 and 40 of embryonic development, and decreased thereafter. During this developmental period, thymic lymphoid cells were shown to progressively transform into smaller cells. A scheme regarding the mode of thymocyte proliferation during the embryonic period is proposed. and the biological meaning of the present phenomenon is discussed.
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