Abstract
The initials of the anulus cells show apical growth. The cell tip is therefore dome-shaped and thin-walled. Eventually this terminal “foramen” is sealed off by a last layer—the swelling layer of the cell — when the apices finally cease to grow. The swelling layer of the mature cell is thus locally in contact with what seems to be the primary wall. Separation of the tissue occurs between this latter wall region and the moderately incrusted secondary wall. Two conclusions were drawn: (1) pressure is exerted through the foramina on lid and urn mouth, and (2) the observed asymmetry of the anulus provides the structural basis for generating shearing stress.
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