Embryos of many bryozoans are retained during development within a helmetshaped brood chamber that is composed of two parts: an outer, double-walled, calcified ooecial fold and an inner, membranous ooecial vesicle. The embryo is brooded “externally” between these two structures and, in Bugula neritina, increases 27 to 35 fold in volume during its embryogenesis. Since the blastocoelic space is obliterated early in development, this change represents an increase in tissue mass. Clearly, some form of extra-embryonic nutrition is implicated. Calvet first noted that the lining of the ooecial vesicle in regions adjacent to the embryo undergoes a pronounced hypertrophy, and Marcus later proposed that this epithelium provides nutrition to the young. Sileh, however, suggested that the hypertrophied layer functions only as a supportive cushion.
