Ultrastructural observations of the incorporation of guinea-pig spermatozoa into zona-free hamster oocytes.

Zona-free hamster oocytes inseminated in vitro with acrosome-reacted guinea-pig spermatozoa were examined with the electron microscope. Guinea-pig spermatozoa, in the vicinity of the oocytes, consistently lacked the whole acrosome including the equatorial segment region. In cross fertilization sperm-egg membrane fusion does not differ significantly from that of normal fertilization. However, it was sometimes possible to observe protrusions of oocyte cytoplasm containing sperm chromatin in the process of dispersion.