Summary

Surface architecture and selected ultrastructural features of normal embryos of the marine gastropod Ilyanassa obsoleta have been investigated using scanning and transmission electron microscopy. A re-examination of the classic account of Anthony Clement on temporal and division patterns in normal and lobeless embryos confirms most of his observations; a few differences, involving the size and positioning of micromere 4d and its derivatives, are noted. Several hypotheses are presented to account for zonular distribution of longer microvilli on some blastomere surfaces and progressive diminution of microvilli on micromere surfaces, especially the leading edge micromeres (LEMs), during postulated epibolic spreading movements. Microfilaments, discovered in the cortical regions of the LEMs, are described, and a role for these structures in gastrulation is proposed.