The Fate of Fertilized Eggs Transferred to the Uterus or Oviduct During Advancing Pseudopregnancy in the Rabbit

Summary. Morulae recovered from superovulated donors 60 hr post coitum were transferred to the uteri of recipients ('temporary' recipients) on Day 3, 5, 7, 9 or 11 of pseudopregnancy. The eggs were again re-covered either 6 hr or 24 hr after transfer and, following microscopic examination, re-transferred to the uteri of synchronous recipients in order to test viability. The proportion of eggs recovered from the temporary recipients, 73 to 97% was little affected by the stage of pseudopregnancy, except for a slight decline on Day 11 compared with earlier stages. However, the reaction of the eggs, as judged after 24 hr, was markedly affected, varying from continued development (up to the early blastocyst stage) in the Day-3 or Day-5 uteri to degeneration in the most asynchronous uteri (Day 9 and Day 11). The appearance of the eggs was strongly correlated with their ability to survive following re-transfer ; practically no eggs (3 out of 113) exposed for 24 hr to the Day-9 or Day-11 uteri survived to term. The associated low rates of pregnancy (failure of implantation) indicated that the eggs perished early (already dead at transfer, or dying shortly afterwards), which was confirmed by autopsy 3 or 4 days after transfer. The injurious effect of the progestational uterus on the morula appeared to develop gradually; it was detectable in some does on the 7th day and expressed itself fully by the 9th to 11th days of pseudopregnancy. No specifically harmful factor or deficiency has been identified in such uteri in which stages earlier than the morula are also affected. Morulae transferred to the oviduct on the 11th day of pseudopregnancy continued to develop with the majority reaching the early blastocyst stage. After 24 or 48 hr under such conditions, 56% and 27% proved fully viable when re-transferred.