||P.577 left column bottom paragraph: "Correlated with the increase in ODC [ornithine decarboxlase] activity, was an accumulation of putrescine and to a lesser extent of spermidine (Fig. 3). Spermine, however, was not present in detectable amounts in early embryos (less than 10 p mol/embryo). Soon after the MBT [mid-blastula transition], the intracellular amounts of putrescine and spermidine ceased to increase and remained at a constant level at least up to gastrulation. The amount of ornithine/embryo increased very slightly over the period studied. To quantitatively correlate the changes in ODC activity and intracellular polyamines, [investigators] determined that the K[m]Orn of Xenopus ODC is 100±10µM and that the ratio of ODC activity measured at 37˚C (temperature of the in vitro assay)/ that at 20˚C (incubation temperature of the embryos) was 3.9±0.2 (data not shown). Using these two values and knowing the amount of intracellular ornithine (Fig. 3) and hence the concentration of ornithine (embryo volume ≈1µl), [they] calculated that during the first 8 h of development the integrated in vivo activity would have converted 0.8±0.1nmol ornithine into putrescine. This value is very close to the measured increase in putrescine plus spermidine over this same period, indicating that the changes in ODC activity, measured in extracts, are a true reflection of the in vivo changes."