An acrosome reaction is prerequisite to sperm penetration of the zona pellucida, but also occurs in immotile sperm. The two types of reaction are difficult to distinguish. However, at the fine structure level vesiculation of the outer acrosomal and overlying plasma membranes was seen as a rather consistent feature of the reaction in sperm that have been incubated for several hours with eggs in tubal fluid in vitro and was only rarely seen in suspensions of fresh epididymal sperm. In either case the acrosome reaction involves loss of most of the outer acrosomal and overlying plasma membranes together with residual acrosomal contents. Detachment of the acrosome occurs along the anterior margin of the equatorial segment. The equatorial segment, which is the most posterior part of the acrosome, remains with the reacted spermatozoon.

As a consequence of the acrosome reaction the perforatorium becomes the anteriormost component of the spermatozoon and the inner acrosomal membrane is exposed as the surface membrane of the anterior region of the sperm. These events have close parallels in several marine invertebrates and may have special significance relative to mechanisms of fertilization.