were 201, 205, and 193, respectively. The nearly equal specific activities indicate that Hb III is synthesized in the duckling and that its presence does not represent simply a carry-over from late embryonic life. The chronology of cell hemoglobin values is shown in Table 1 . The times of the activation and deactivation of hemoglobin synthesis have not yet been determined. Cell hemoglobin content is maximum in the 7-to 9-day embryo. The subsequent decrease at 14 days is conincident with the

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... ppearance of the so-called "primitive" cells (8). The reciprocal change in cell content of Hb's I and II stabilizes in the adult, while Hb III decreases and ultimately disappears. We suggest that the appearance of Hb III in the duck embryo, its presence in the duckling, and its disappearance in the adult are similar to characteristics of fetal hemoglobin in the human. According to current epigenetic concepts, the inactivation of genes for duck embryonic hemoglobins occurs at widely separate times, that for Hb IV early in embryonic life and that for Hb III after hatching.