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catesbeiana tadpoles, hemoglobin synthesis in the circulating red blood cells initially declines to very low levels . This period is followed by the synthesis of a new hemoglobin which is characteristic of the adult bullfrog. Radioautographic studies on smears of whole blood cells have indicated that the production of adult hemoglobin is associated with the appearance of a new cell type which differs from mature tadpole blood cells in size and shape (Moss and Ingram, 1965 ; 1968 a and b ; DeWitt, 1968) . Recently it has been shown that the thyroxin-induced switch in hemoglobin synthesis is also accompanied by striking changes in the synthesis of erythrocyte ribosomal RNA . These include an initial breakdown of ribosomal RNA followed by an increase in RNA synthesis at the time of appearance of frog hemoglobin (McMahon and DeWitt, 1968) . It was expected that biochemical changes occurring during the switch in hemoglobin synthesis could be correlated with changes in cell morphology. Ultrastructural studies were, therefore, initiated to examine the cell types present in the peripheral blood following the administration of thyroxin to bullfrog tadpoles . These results clearly demonstrate the existence of two distinct red blood cell types which appear to differ significantly in their synthetic activities .

ULTRASTRUCTURAL STUDIES OF RED BLOOD CELLS FROM THYROXIN-TREATED RANA CATESBEIANA TADPOLES