AbstractThe ultrastructure of blastocoelic material in blastulae and gastrulae of the sea urchin Lytechinus pictus was revealed by transmission and scanning electron microscopy.The blastocoel of blastulae contained 30 nm diameter granules, aggregated in long chains. In gastrulae, on the other hand, another component was present: fine fibers. The 30 nm granules were attached to the surface of these fine fibers to form a thicker, rough‐surfaced complex fibrous structure.Embryos cultured in sulfate deficient sea water had greatly reduced numbers of 30 nm granules in the blastocoel. The fine fibers appeared normal. Consequently, the 30 nm granules are selectively sensitive to sulfate deprivation.