OF ALL THE developments and disclosures produced by glasnost in the Soviet Union and by its precursors and spinoffs in Soviet-dominated Eastern Europe, perhaps none is more dramatic than a statement made in Warsaw the other day by a spokesman of the Polish government. He said the Soviet Union was responsible for the massacre of Polish officers at Katyn, near Smolensk in the Soviet Union, early in World War II. Katyn. The word carries immense emotional and nationalistic freight for Poles. The Soviet dictator Stalin, in the course of subduing the part of Poland the Soviet Union grabbed in its infamous pact with Nazi Germany in 1939, swept up some 16,000 Polish officers -- a good part of the cream of the nation's leadership, Poles always felt. The German forces that overran Katyn while later invading the Soviet Union found the bodies of 4,443 of these officers in mass graves, each with a bullet hole in the back of his head. The Germans said the Soviets had killed the officers. The Soviets said the Germans killed them. The available evidence has always indicated that in this case Hitler was telling the truth and Stalin was lying. Successive Soviet leaders kept on lying, and Poles, who knew better, were compelled to go along. By 1987, glasnost and reform had moved far enough along to permit Mikhail Gorbachev and Wojciech Jaruzelski, the Soviet and Polish communist leaders, to set up a joint commission to examine this and other ''blank spots'' in their common history. Everyone realized how sensitive it would be to examine an instance of Soviet mass murder of Poles. The Soviets have still not reached the matter of Katyn in their own published accounts. The Polish authorities, however, presumably grasping how fundamentally important it is to their credibility to satisfy national feeling on this transcending question, have now publicly pointed a finger straight at Moscow on Katyn. This is far from the only ''blank spot'' that needs to be painted in with the truth. But the accurate assignment of responsibility for the Katyn massacre is an essential contribution to the rediscovery of dignity in East Europe and to the termination of a regime of killings and lies.