Development of horse embryos up to twenty two days after ovulation: observations on fresh specimens.

Forty nine embryos, twenty unfertilized eggs and five other fresh eggs of 'doubtful' status have been recovered from 58 pony mares in 122 flushes up to 22 days after ovulation. The fresh egg or embryo recovery rate was 78% with surgical methods (or at slaughter) and 40-60% with non-surgical methods of recovery. The fertilization rate was about 70%. It has been confirmed that horse embryos normally enter the uterus as blastocysts 5-6 days after ovulation. Three features of early embryo morphology have become clearer upon comparison with unfertilized eggs of similar ages; early embryos are often ellipsoidal in shape; dispersal of most of a thick gel coat seems to be hastened by fertilization; gradual disappearance of refractile granules from the perivitelline space is similar in fertilized and unfertilized eggs. A tense, transparent, acellular capsule (considered to be different from the zona pellucida) is acquired by the spherical blastocysts within the uterus and persists at least until a diameter of 34 mm is attained (at 21 days in the present series). The capsule seems to be analogous, in part, with the 'neozona' described in rabbit blastocyst before attachment, and trophoblastic cells appear to be involved in its formation. Cleavage stages of oviductal embryos and diameters of uterine blastocysts from this series have been described and illustrated and used to extend previous knowledge of early growth patterns in horse embryos.