Eggs were examined from a control group of gilts (Group 1) which were inseminated at least 6 hr before ovulation, and from five other groups (Groups 2 to 6) in which the postovulatory age of the eggs was estimated to be 4, 8, 12, 16 or 20 hr at the time of sperm penetration. Penetrated eggs were obtained from twelve animals in each group.
The conception rate fell from 100% in the control group to 66·7% in Group 6, whereas the incidence of unilateral fertilization increased from 8·3% in the controls to 33% in Group 6. Within the seventy-two animals with some penetrated eggs, the mean percentage of unfertilized eggs increased from 1·3% (Group 2) to 23·9% (Group 6), but unfertilized eggs occurred principally as a result of unilateral fertilization.
No decrease in the mean proportion of fertilized eggs developing normally was found until the eggs were 12 hr old at penetration (Group 4), when the figure fell to 73·6%. A further decrease occurred in Groups 5 and 6, means of 55·4% and 66·9% respectively, being obtained. The mean percentage of polyspermic eggs increased from 1·3% in Group 2 to 15·4% in Group 5, and the proportion of eggs showing varying degrees of fragmentation also increased from 3·8% in the control group to 29·2% in Group 5.
The results indicate that a very high proportion of eggs can be fertilized up to 8 hr after ovulation and undergo apparently normal development at least until the 4- to 8-cell stage.
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