[Morphogenetic role of the cortex in the development of twins in sea urchins]. The dynamics of development of Strongylocentrotus nudus sea urchin twins has been studied after centrifugation at 2,000 g for 5 min and incubation in hypotonic medium after fertilization till formation of the first cleavage furrow. Cortex rigidity was found to correlate with the frequency of twins occurrence. The largest amount of twins forms after centrifugation and hypotonic treatment for the first 1-6 min of zygote development. The twins may be obtained at 25-40 min of development by centrifugation in the presence of cytochalasin B (1 micrograms/ml). Morphogenetic role of cortex in development of sea urchin twins has been discussed.