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THE DEVELOPMENT OF SHEEP EGGS IN THE RABBIT OVIDUCT AND THEIR VIABILITY AFTER RE-TRANSFER TO EWES

Summary.: A total of 456 sheep eggs at the two-, four-, eight- and sixteen-cell stages were transferred to the ligated oviducts of pseudopregnant or oestrous rabbits and 397 (87%) of the eggs were recovered. Of these, 368 (93%) continued development in the rabbit. The proportions of eggs recovered and of eggs developing were not affected by the cleavage stage at the time of transfer to the rabbit or by the endocrine status of the rabbit. Development in the rabbit oviduct appeared to proceed normally up to the blastocyst stage. In many instances, although rupture of the zona pellucida occurred, complete escape from it appeared to be frustrated by the `mucin coat' acquired in the rabbit's oviduct.

Sheep eggs recovered from the rabbit's oviduct were re-transferred to ewes which had been in oestrus within 24 hr of the original donor ewes. The highest survival rates to lambing were obtained when eggs were re-transferred to recipient ewes after 3 days in the rabbit. The best rate of survival (69%) was with two- and four-cell eggs which had been in pseudopregnant rabbits for 3 days.

The proportion of the re-transferred eggs which survived declined when the period in the rabbit oviduct was extended from 3 to 5 days, and was negligible after 7 days. The results are discussed in relation to the pattern of embryonic death observed in the recipient ewes.

In order to test whether the lessened viability of such eggs resulted from retardation of their development, sheep eggs recovered from oestrous rabbits were also re-transferred to recipient ewes which had come into oestrus 48 to 60 hr later than the original egg donors. The survival rate of such eggs was not improved.

A further fifty-one one-cell eggs recovered from donor ewes on Day 2 were transferred to the ligated oviducts of oestrous rabbits. All of the eggs in which there had been some evidence of fertilization continued to develop, some as far as the blastocyst stage.

It was concluded that the oviducts of either oestrous or pseudopregnant rabbits provide a satisfactory environment for the development of sheep eggs from the one-cell stage blastocysts, but the viability of such eggs declines rapidly after more than 5 days in the rabbit.