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Summary. Two experiments are described in which were studied the morphological appearance and the subsequent development of sheep ova fertilized by uterine insemination. Ewes were naturally mated, or inseminated with freshly ejaculated semen, either whole or following removal of the seminal plasma, or with semen recovered from the uteri of naturally mated ewes ('uterine semen'). Fertilized eggs were collected after insemination or mating and were either stained and examined or transferred to recipient ewes. Uterine insemination with whole or fractionated semen gave very high rates of fertilization. 'Uterine semen' gave a low fertilization rate due to poor quality samples. The proportion of eggs fertilized by uterine insemination which developed into lambs or embryos was similar to that of eggs collected from ewes naturally mated. Treatment of semen before insemination had no effect upon subsequent development of fertilized eggs. More than 50 % of the fertilized eggs contained anucleate particles, but neither the method of mating (uterine insemination or natural mating) nor treatment of semen before insemination had any major effect upon the incidence of such eggs. The presence of anucleate particles did not preclude subsequent normal development. It is concluded that the low lambing percentages recorded following uterine insemination are not due to fertilization failure or abnormalities of fertilization, but to surgical interference with the tract resulting in expulsion or rapid transport of fertilized eggs.

THE MORPHOLOGICAL APPEARANCE AND DEVELOPMENT OF SHEEP OVA FERTILIZED BY SURGICAL INSEMINATION