Heterocellular transgene expression. Circulating blood from d10.5 yolk sacs (A-B) or a transverse section through a d13.5 embryo (C) demonstrates a mosaic pattern of transgene expression in positive transgenics (B-C). Also shown is a negative control littermate (A). (A-B) Large diffracting cells are primitive erythroid cells from the embryo, and small concave cells are adult definitive cells from the mother. liv indicates liver. Original magnification, × 100 (A-B); and × 75 (C).