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The Fertilisation of Rabbit Ova in Relation to Time : A Method of Controlling the Litter Size, the Duration of Pregnancy and the Weight of the Young at Birth
1 School of Agriculture, University of Cambridge
1. By matings with fertile bucks at different intervals of time after a mating with a vasectomised buck (to induce ovulation) the time of insemination in the rabbit can be moved successively nearer to, at, and after the time of ovulation.
2. From inseminations 5 hours before to 2 hours after ovulation the percentage of matings which are fertile, the average litter size, and average number of young permating fall gradually and at a constant rate until absolute sterility is obtained. Does which produce large litters when mated normally (10 hours before ovulation) may be made to produce small litters by mating shortly before or after the time of ovulation.
3. The ova are only capable of fertilisation while they are in, or as they leave, the plug at the top of the Fallopian tubes, i.e. up to about 6 hours after ovulation. They are incapable of fertilisation as they begin to move down the tube and acquire a layer of albumen around them.
4. As a result of a mating made about the time of ovulation the apex of the "sperm swarm" arrives at the top of the tube in time to fertilise only those ova which still remain in the plug. The apex of the "sperm swarm" contains relatively few spermatozoa, and so small litters are produced.
5. By "double matings," of dominant and recessive coloured bucks on recessive coloured does, with a time interval between them, it is possible to measure the relative sizes of the "sperm swarms" at the tops of the tubes at different intervals of time after mating.
6. The size of the "sperm swarm," and so the interval of time over which it is possible to obtain fertilisation as a result of mating later than normal in relation to the time of ovulation, varies considerably in different males.
7. The speed at which the sperms ascend the female tract is apparently not affected by the presence in them of male and female sex-determining elements.
8. The duration of pregnancy is prolonged by reducing the size of the litter. With the prolongation of pregnancy the proportion of young born dead increases. Various possible causes for these facts are discussed.
9. While there is but little constant increase in the weight of the individual young due to the prolongation of pregnancy from 31 to 34 days, there is a large increase in the individual weight due to reduction in the number of young in the litter.
10. It is probable that the size of individual young is limited by the amount of some internal secretion or metabolic product of the mother.
Submitted on May 22, 1933
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