Direct Evidence of Retention of Unfertilized Ova in the Oviduct of the Mare

Van Niekerk & Gerneke (1966) first showed that in mares several unfertilized ova can be found in the oviduct at any one time. This has been confirmed by Webel, Ellicott & Dziuk (1970), Thompson (cited by Allen, 1970), Betteridge & Mitchell (1972) and Steffenhagen, Pineda & Ginther (1972) and has also been shown to occur in donkeys (Bielanski & Zapletal, 1968). It seems highly probable that these ova originate from previous normal cyclic ovulations and persist in the oviduct for several months, but the evidence for this has so far been circumstantial in that persistence of individually marked ova has not been followed. During studies on the origin of accessory CL, we obtained direct evidence in two mares that individual ova were retained in the oviduct for