“…The effect of crowding on supernumerary eye-formation is attributed to ammonia excreted from the planarians (Kanatani, 1957b). Abnormal eye-formation in regenerating heads is observed in planarians treated with some drugs: p-chloromercurbenzoic acid (Smith and Hammen, 1963), lithium chloride (Brøndsted, 1942;Kanatani, 1958) and thyroxine (Mengebier, 1961). These studies of crowding and drug effects were performed to perturb the physiological states of planarians, and did not focus on the developmental aspects of eye formation.…”