Formation of Sperm Entry Holes in the Vitelline Membrane of Hydroides hexagonus (Annelida) and Evidence of their Lytic Origin

Abstract

Electron micrographs of inseminated eggs of H. hexagonus previously had shown that in the immediate vicinity of the penetrating spermatozoo''n a small portion of the vitelline membrane regularly was absent, and it had been suggested that this area was a hole made by lytic activity of the individual spermatozoo''n during the course of its passage through the membrane. This deduction would receive support if it could be established that a sperm entry hole does form in living material. During the present study a hole repeatedly observed and photographed in the membrane of living eggs was found to arise as the spermatozoo''n penetrated the membrane. Gently compressed eggs formed exovates only through this hole. The holes, and exovates, were not found except at sperm entry sites. It was concluded that this hole is the counterpart of the area from which the membrane is absent in the electron micrographs cited above, and that the spermatozoo''n makes this hole. In an electron micrograph 2 spermatozoa which had penetrated the membrane at separate but closely neighboring points now occupy a single hole. It is argued that if each spermatozoon had displaced the membrane mechanically to make its hole, then there should be 2 holes, with a partition of membrane between them, but if each had eroded the membrane by applying lysin, a single hole should have formed as the eroded areas expanded and finally merged into 1. The latter view agrees with the facts of the electron micrograph. It is concluded that lysis is the most probable means by which the individual spermatozoo''n makes its hole.