In this Chapter we shall examine the experimental embryology of some diverse types of animal: molluscs, annelids, ascidians, and nematodes. These groups can be collectively contrasted to the embryos considered in the previous Chapter in so far as the key decisions of early development seem to be made at a very early stage when there are only a few cells in the embryo. This may mean that each individual cell has a unique identity in terms of its biochemical properties and corresponds to a zone of tens or hundreds of similarly committed cells in a vertebrate, insect, or sea urchin embryo. The small cell number means that all individual embryos of a given species are identical or nearly identical and makes it possible in principle to construct fate maps of very high precision by direct observation of the cell lineage. A number of studies of this type were carried out around the turn of the century and some, such as Wilson's (1892) study of Nereis, Wolterek's (1904) study of Polygordius and Conklin's (1905a) study of Styela, are masterly works still referred to today. More recently the introduction of Nomarski interference contrast microscopy has made it possible to extend this approach to the nematode Caenorhabditis and to carry it to the ultimate limit of a complete cell lineage from egg to adult (Sulston et al., 1983).