The ovicells are modified zooecia, and contain numerous embryos which in the cases so far investigated arise by fission of a primary embryo developed from an egg.
Zooecia either arising from a stolon, without lateral connexion with one another, or laterally united to form sheets.
The alternate increase and diminution of volume is easily understood in forms with flexible zooecia.
As indications that the conditions described in Membranipora and Cribrilina are of special significance may be noted the fact that the ancestrula of many genera which have well-developed compensation-sacs in the rest of their zooecia is a Membranipora-like individual with a series of marginal calcareous spines, and the further fact that a considerable proportion of the Cretaceous Cheilostomes belong either to the Membraniporidae or to the Cribrilinidae.
In the majority of Cheilostomes, the avicularia are, so to speak, forced out of the ordinary series of zooecia, with which they are rigidly connected.