Implantation
The embryo enters the uterus around day 5-6. Nutritional support is provided by the yolk sac and ‘uterine milk’. The embryo is fairly mobile up until day 16-17, when the combination of a metabolite production and increasing tone of the uterus ‘fixes’ the embryo, usually at the base of either uterine horn. Growth of the embryo plateaus around day 24 while the fetal membranes develop but returns to rapid growth around day 28.
Placentation
The horse has a specialized yolk sac placenta which provides most of the support for the first 40 days of the concepsus’ life. A chorioallantoic placenta then takes over. The placenta is made up of 2 distinct layers: Amnion: the opaque membrane which directly surrounds the foal. It is filled with amniotic fluid. Chorioallantois: which is attached to the endometrium of the uterus except at the cervical star. The outer chorionic surface is the velvety layer while the inner surface is the glistening allantoic layer. It is filled with allantoic fluid (water breaking). When the foal is born the placenta inverts. The foal, still enveloped in the amnion, breaks through the chorioallantois at the cervical star. The foal moves into the cervix and vagina pulling the chorioallantois by way of the umbilical cord, everting it through the hole at the cervical star.
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