The ascidian embryo has long provided a model system for ‘mosaic’ development. This article reviews recent advances in the study of ascidian developmental biology. These include: (a) the re‐analysis of cell lineages in ascidian embryos with the ascertainment of developmental fates of every blastomere of a 110‐cell embryo; (b) the development of several tissue‐specific monoclonal antibodies; (c) the investigation and description of cell cycle requirements for differentiation; it has been found that neither cytokinesis nor nuclear division is required for differentiation, (...) but that several rounds of DNA replication are essential for the expression of certain tissue‐specific genes; and (d) the demonstration by new descriptive and experimental studies of the presence of cytoplasmic factors or determinants responsible for specification of embryonic cell features; myoplasm which is thought to contain muscle determinants has been isolated, and immunological attempts to elucidate the molecular nature of the factors have begun. (shrink)