Summary. The fate of embryos transferred asynchronously in the ewe was investigated when the functional life of the corpus luteum was prolonged by both hemi-hysterectomy and by the presence of a second synchronously transferred embryo. The development of asynchronously transferred embryos was assessed at progressively later stages after transfer. Prolongation of luteal function did not enable asynchronously transferred embryos to persist. Embryos from Day 4 donors were found to be retarded in their rate of development when placed in 'younger' Day 1 or 2 uteri and appeared unable to develop beyond the early blastocyst stage. Conversely, embryos from Day 4 donors placed in 'older' Day 6 or 7 uteri showed accelerated growth and development which was maintained until the uterus reached Day 12. Thereafter further growth of the asynchronously transferred embryos was retarded, although synchronously transferred embryos then entered the phase of rapid blastodermic vesicle elongation. Asynchronously transferred embryos disappeared from the uterus when the ewe entered pro-oestrus.
The experiments demonstrate the existence of an active relationship between the embryo and the maternal environment during mid-cycle and an apparent lack of association between embryo size, growth rate and physiological maturation.