Echinoid embryos of indirect developers comprise three tiers at the 16-cell stage: eight mesomeres, four macromeres and four micromeres. Micromeres differentiate autonomously into skeletogenic mesenchyme cells and induce adjoining macromeres to form a vegetal plate, resulting in archenteron invagination (Ransick & Davidson, 1995). It has been shown that micromeres have the potential to induce archenteron differentiation of presumptive ectoderm (Hörstadius, 1939).
Direct development, in which the larval stages are more or less abbreviated, has evolved in several phylogenetic lineages of echinoids. In most of them, the fourth cleavage of the embryos produces 16 blastomeres of almost the same size (Raff, 1987). Because of their indistinguishability, the inductive potential of vegetal-most blastomeres of direct developers remained to be studied.
The 16-cell stage embryos of the direct-developing sand dollar, Peronella japonica, are an exception and produce three tiers, the same as indirect developers. The embryos develop into two-armed pluteus-like larvae and metamorphose within 3 days after fertilisation without feeding. The micromeres of the species had been shown to differentiate into the skeletogenic mesenchyme cells (Amemiya & Arakawa, 1996). In the present study, we investigated the potential of the micromeres to induce or promote archenteron invagination in Peronella japonica.