Persistence and parthenogentic cleavage of tubal ova in the mare

Abstract

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.