The Collaborative International Dictionary of English v.0.48:

Ovisac \O"vi*sac\, n. [Ovum + sac.] (Anat.)
   (a) A Graafian follicle; any sac containing an ovum or ova.
   (b) The inner layer of the fibrous wall of a Graafian
       follicle.
       [1913 Webster]