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Cytological observations on sperm penetration of rabbit eggs.

Photomicrographs of the cytological changes of the sperm head and of the female chromatin in the rabbit egg, from the time of sperm penetration and activation of the second meiotic metaphase up to the formation of pronuclei and syngamy, are presented. During a study on factors affecting sperm penetration of rabbit eggs, some anomalous eggs were observed. One unfertilized egg possessed two second meiotic metaphases and associated first polar bodies, while another similar egg had two first and two second polar bodies, two sets of female chromatin, and one fertilizing sperm. In this latter egg fertilization was apparently proceeding normally. Two eggs within a single zona pellucida were also seen; one of these was a primary oocyte, while the other was a normally penetrated egg. Evidence is also presented which indicates that the incidence of polyspermy (usually dispermy) is increased, if eggs enter the oviduct at 14 hours or later after mating. This is thought to be due to more fully capacitated sperm present at the site of fertilization at these later times, and thus there is a greater chance of two or more sperm entering the vitellus at the same time.