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

The phenomenon termed umbilication, common to the vaccinal pock and to that of variola, has given rise to some differences of opinion as to the mechanism of its production.

This umbilication in the vesicular stage is somewhat peculiar.

This is sometimes termed a secondary umbilication.

The umbilication of the mature pock is doubtless due to the situation of such lesions at the orifices of the excretory ducts of the skin-glands.