The distribution of extracellular material (ECM) in developing neuroepithelium of stage 8+ chick embryos was investigated using lanthanum nitrate. The temporal changes in the distribution of ECM was correlated with the ongoing morphogenetic movements. The most striking finding was that as the folds were about to meet, thick dense ECM appeared between leading edges. Concanavalin A inhibited neural tube closure by binding to cell surfaces and disrupting the usual distribution of ECM in developing neuroepithelium.