oöcyst

from The Century Dictionary.

• noun A unicellular structure of the algæ, producing egg-cells: same as carpogonium. See oöangium.
• noun In sporozoans, the tough, resistent wall surrounding a zygote.
• noun In zoology, an ovicell; a sac or pouch serving as a receptacle of the eggs of certain polyzoans, to the cells of which it is attached; a kind of oötheca or oöstegite.
• noun In botany, same as oögonium.