[Invagination and evagination: a hydromechanical model]

It has been proposed that processes of invagination and evagination be considered in the context of a single, "hydromechanical," model. The model is based on the assumption that the structures demonstrating invaginations and evaginations are closed systems capable of changing their intracavitary pressure in an autonomous regime. The mass, which occupies the cavity, should overcome the resistance of the surrounding membrane during its growth. In places of the weakest resistance, the inner mass expands especially quickly: it bulges out. If the pressure under the membrane is less than the exterior one, the process goes in the opposite direction: the outer mass intrudes (invaginates) into the cavity, causing its wall to sag in the weakest regions.