1. Isolated blastomeres and pairs of blastomeres from 8‐cell embryos of Halocynthia roretzi and Halocynthia aurantium were cleavage‐arrested with cytochalasin B and cultured. Their differentiation was examined in terms of membrane excitability, immunoreactivity to an epidermis‐specific monoclonal antibody (2C5), and the presence of acetylcholinesterase. 2. The blastomeres that showed epidermal‐type differentiation had Ca2(+)‐dependent action potentials and membrane currents, and immunoreactivity to 2C5. The blastomeres that showed neural‐type differentiation had Na(+)‐, Ca2(+)‐ and TEA‐sensitive delayed K+ channels, and lacked immunoreactivity to 2C5. 3. Cleavage‐arrested anterior‐animal blastomeres, a4‐2, when cultured in isolation from an 8‐cell embryo, differentiated exclusively into epidermal‐type cells. However, when cultured in contact with anterior‐vegetal blastomeres, A4‐1, they mostly showed neural‐type differentiation (seventeen out of twenty‐four cells in H. roretzi). 4. Reduction of the cytochalasin B concentration enhanced neural‐type development of a4‐2 blastomeres in contact with A4‐1 blastomeres in H. aurantium, possibly by tightening the physical contact between the blastomeres. 5. When a cleavage‐arrested and isolated a4‐2 blastomere was treated with 2% pronase at 10 degrees C for 15 min at the time when sister control embryos reached the 32‐cell stage, the blastomere underwent neural‐type differentiation in a manner identical to that of a4‐2 blastomeres contacted by A4‐1 cells. 6. The period during which neural‐type differentiation of a4‐2 blastomeres could be induced by treatment with pronase was from the 8‐cell to the 110‐cell stage. At the late gastrula stage neural‐type differentiation of a4‐2 blastomeres was not induced by pronase. The effective period for neural‐type differentiation of a4‐2 blastomeres in contact with A4‐1 cells was between the 64‐cell stage and late gastrula stage. Competence of the a4‐2 blastomere to undergo neural‐type differentiation decreased during gastrula stages, while the inducing ability of the A4‐1 blastomere lasted longer. 7. In a few cases the posterior‐animal blastomere, b4‐2, could also be induced to undergo neural‐type differentiation after contact with A4‐1 cells or after pronase treatment. 8. The appearance of Na+ spikes in a4‐2 blastomeres in contact with A4‐1 cells was considered a manifestation of neural induction, similar in principle to the induction of ectoderm by the chorda‐mesoderm in higher vertebrates. © 1990 The Physiological Society
