The fine structure of rat thymic lymphocytes from early prophase to late telophase of mitosis is described, using material fixed at pH 7.3 either in 1 per cent OsO4 or in glutaraldehyde followed by 2 per cent OsO4. The structure of the centriolar complex of interphase thymocytes is analyzed and compared with that of centrioles during division. The appearance of daughter centrioles is the earliest clearly recognizable sign of prophase. Daughter centrioles probably retain a secondary relation to the primary centriole, while the latter appears to be related, both genetically and spatially, to the spindle apparatus. The nuclear envelope persists in recognizable form to help reconstitute the envelopes of the daughter nuclei. Ribosome bodies (dense aggregates of ribosomes) accumulate, beginning at late prophase, and are retained by the daughter cells. Cytokinesis proceeds by formation of a ribosomefree plate at the equator with a central plate of vesicles which may coalesce to form the new plasma membrane of the daughter cells. Stages in the formation of the midbody are illustrated.

Thymocytes of young rats were studied with the electron microscope to provide a base line for a comparative study of the fine structure of various types of lymphocytes. Fixation with Os01 or glutaraldehyde, Epon embedding, and heavy metal staining were the principal techniques employed. Small thymocytes, with nuclei from 3.5 to 4.5 . u in diameter are in the vast majority. Our results, which in general confirm previous reports, show the thymocytes to be primitive, or undifferentiated in structure. Certain exceptions and additions were noted, however. There are no protoplasmic bridges between thymocytes and epithelial cells and no clear evidence of epithelial-thymocyte transformations. The nucleus contains typical pores and in many cases a clearly defined finely granular nucleolus, although the latter is not evident in most of the smallest cells. Cytoplasmic fragments appear to be separating from thymocytes by a process similar to the separation of platelets from megakaryocytes. The existence of a special band of cytoplasm beneath the plasma membrane devoid of formed elements is described for the thymocytes. The possibility of the common occurrence of this zone in other cells, and its possible significance, are discussed.The past few years have seen a revival of interest in the problem of the nature and fate of the small cells of the thymus, and particularly as they may relate to the lymphocytes of the blood and hemopoietic tissues (Arnason, '62; Miller, '63). Part of the difficulty arises from the fact that the lymphocyte is not one clearly defined cell, but a family or species, varying from small cells composed almost entirely of dense nuclei, at one extreme, to large cells with abundant cytoplasm and relatively open nuclei at the other. These latter cells have been called by a variety of names (stem cell, lymphoblast, hemocytoblast, etc.) indicating a variety of opinions regarding their relation to the lymphocyte family and to other blood cells. In addition, the lymphocytes of the blood are connected to the monocytes by an unbroken series of transitional forms (Bloom and Fawcett, '62), and in the spleen and elsewhere the lymphocytes form a series with the plasma cells (Nossal and Makela, '62). There is evidence that under certain conditions the lymphocyte can act as a stem cell for granular leukocytes (Bloom, '37; Farr, '51) and possibly also erythrocytes (Yoffee, '60), and that thymocytes may have similar potential (Murray and Woods, '64). In the thymus the free cells present a series which is morphologically identical with the range seen in lymphatic tis-ANAT. REC., 1 5 1 : 17-40. sues, but there is very little evidence of in situ transformation to other cell types. Another factor is present here, however, which adds to the uncertainty. It is not clear whether the thymocytes originate from the local stroma of epithelial nature, or from mesenchymal elements which have invaded this stroma. The weight of evidence has favored the latter hypothesis (Maximow, 'Og), but recent data seem to point to the...