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The observations presented in the previous section represent the present state of the art of mapping neutral hydrogen in external galaxies. Aperture synthesis observations of a number of nearby late-type spiral galaxies are in progress at the present time, and we should soon be able to see if the HI distributions of, say, Sb and Sc galaxies are characteristically different.

Maps made with the Westerbork telescope to an angular resolution of 23 seconds arc should soon be available, allowing a detailed comparison of HI structure in many more galaxies. Details of the HI density and velocity distribution in the luminous spiral arms should make possible a much better understanding of star formation and of the dynamics of the spiral patterns.