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Empty cells becoming confluent, vesicular by incurvation of the circular margin, forming a deep and round umbilication.

Early in their career an apicial depression can be seen, which later deepens into a characteristic umbilication.

This umbilication in the vesicular stage is somewhat peculiar.

It is pea-sized and globular in shape; its umbilication has been usually quite removed by the complete filling of its chamber with distinctly purulent contents; it is often surrounded by a halo due to hyper�mia or exudation; and, the total number of individual lesions being then fairly determined, it is often closely set against its fellows, islets of unaffected integument having meantime become fewer and more contracted.

Others desiccate en masse, the roof-wall first collapsing upon the contents, thus producing an appearance which again suggests umbilication of the lesions.