The intact perivitelline layer (PL) at ovulation was composed of a meshwork of anastomosing fibers uniformly enveloping the hen’s ovum. When incubated under appropriate conditions with fresh cock sperm areas of this reticulum were hydrolyzed presumedly by acrosin. Light microscopy and scanning and transmission electron microscopy revealed PL breakdown to be considerably more extensive over the animal pole. The sperm observed embedded in this hydrolyzed matrix did not exhibit a typical mammalian or invertebrate acrosome reaction. The plasmalemma overlying the nucleus in most sperm was distended and the acrosomal membranes appeared fenestrated. The plasmalemma overlying the acrosome in a few sperm was discontinuous. Under identical experimental conditions, ova removed from the distal infundibulum or proximal magnum of the oviduct fail to show PL hydrolysis. It appears that the tertiary layers may mask PL sperm receptor sites, act as physical barriers not rendered penetrable by cock sperm acrosin or contain acrosomal enzyme inhibitors.