Bovine oocytes were partly denuded either at the beginning (to) or six hours (td after the beginning of maturation and vitrified by the open pulled straw method at the end of the maturation process. After warming and fertilisation, their development in vitro and in vivo was assessed. The rates of production of blastocysts achieved in vitro were 3-4 per cent for the to group and 0-9 per cent for the t6 group compared with 40-4 per cent for the control oocytes. After transfer at the blastocyst stage pregnancies have been established in the three groups. Some of these pregnancies originated from vitrified oocytes which were further vitrified at the blastocyst stage before being transferred into synchronised recipients.
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