As many as ten different ova in all stages of cytolysis were encountered in the
uterine tubes of mares. As the follicles of the mare were consistently found to be
mon-ovular, it is concluded that unfertilized ova do not, as a rule, pass out of the
uterine tubes, but, contrary to all accepted data on the migration of ova, may remain
there up to seven and a half months or longer. During this time they undergo
gradual disintegration characterized by the following order of changes: deutoplasmic
condensation, deutoplasmic extrusion, cytoplasmolysis, deutoplasmic fragmentation
and comminution of yolk granules. The final stage identified is a fluid-filled, collapsed
vesicle (zona vesicle) surrounded by the zona pellucida only.
Immediately after ovulation the ovum was found to be without a corona
radiata but enclosed in a large, irregular gelatinous mass of follicular origin. This is
considered to be the reason why newly ovulated ova are so often missed. This mass
becomes separated from the ovum within the second day after ovulation. Its significance
in fertilization is speculated upon.
Some unfertilized ova are able to undergo parthenogenetic cleavage. This
hampers the identification of early cleavage stages of fertilized ova.
Ovulation takes place after the first meiotic division. Sperm entrance stimulates
the second meiotic division, as in most mammals.