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Fig. 1-2. Photograph of thyroid tissue from a fetus with a 50-mm crown-rump length, estimated gestational age 64 days. The arrows indicate two intracellular canaliculi. During incubation of the tissue in organ culture in vitro, there was no uptake or fixation of iodide. The figure shows the earliest stage of formation of colloid spaces. The tissue was fixed in ozmium, embedded in Epon epoxy resin, and sectioned at 1-mm thickness (x2,400). (From T.H. Shephard, J. Clin. Endocrinol. Metab., 27:945,1967, with permission).