The sperm:egg ration and sperm concentration were varied separately by using different volumes (1--100 microliters) and egg numbers (1--15). The % fertilization was determined by sperm concentration in large volumes (10--100 microliters). In small volumes (1 or 5 microliters) the number of spermatozoa/egg determined penetration rate: the effective epididymal sperm number/fertilization was 250--350. Spermatozoa obtained from the oviducts (i.e. naturally selected) gave good fertilization responses and low concentrations of epididymal spermatozoa were also more effective when oviducal fluid and cumulus cells were included in the inseminating volume. We conclude that sperm/egg collision rate is the most important factor determining % fertilization, although heterogeneity of epididymal spermatozoa limits fertilization rate at low sperm:egg ratios.

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