Sixty fertilized sheep ova were stored at 7° C in sterile sheep serum for either 72, 96 or 120 hr. Thirty of these ova were between two and six cells and after storage were transferred to the Fallopian tubes of fifteen recipient ewes. The remaining thirty ova were at the eight-cell stage or older and after storage were transferred to the uteri of fifteen recipient ewes. In each case two ova were transferred to each ewe. None of the ova survived these procedures and no lambs were born. It was observed that in the groups of ewes receiving ova which had been stored for the shorter periods, four ewes failed to return to oestrus within the normal cycle limits. It was suggested that this might indicate that some of the ova had `implanted' but failed to survive to term.
Data are also presented on the ovulatory response of Welsh mountain ewes to a standard dose of pregnant mares' serum gonadotrophin (pms) when given on the 12th or 13th day of the oestrous cycle. Some observations on the stage of development of the ova in relation to the time elapsing between the onset of oestrus and recovery are included.
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