The rate of uptake of [14C]valine and [3H]cytidine into the metabolic pool of early embryos of the sea urchin Paracentrotus lividus was measured by giving 5-min pulses of these precursors. The uptake rate rose sharply after fertilization, reached a maximum before first cleavage and then stayed constant until at least the fifth interphase. There were no changes in uptake rate at cell division. A longer-term experiment with valine showed that the uptake rate remained approximately constant until the pluteus stage, and was not altered for the first 10 h by a actinomycin D. Continuous labelling experiments indicated that the uptake rate may be controlled by the level of precursor in the pool. The following experiments indicated that the uptake was by specific transport mechanisms. Both precursors showed an uptake inhibited by competitors, an apparent concentration within the cells, and no back-exchange. In addition, the uptake of valine was inhibited by dinitrophenol and azide, and showed a concentration response with Michaelis-Menten kinetics. Valine uptake was not inhibited by puromycin added at fertilization, though it was somewhat affected if the eggs were pretreated.

It is suggested that the transport mechanisms are mediated by specific carrier molecules at the cell surface. These mechanisms are activated at fertilization through links with metabolism. There is no change in the number of carriers per embryo throughout early development. Since the number of cells and the total cell surface are increasing, this implies that the number of carriers per cell or per unit of surface is decreasing. There is a possible reason for this differentiation of the cell surface.

Post-Doctoral Fellow of the National Institutes of Health, U.S.A. (2-F2-GM-18, 755-02).