У аллантоиса есть внезародышевая часть. Она как раз и формирует "красную сумку".
Вот по привычке на Яндекс вышла и тут он себя реабилитировал.
Вот она, "красная сумка":
Вот подробнее о нем. Правда, тоже по-английски:
Premature separation of the chorio-allantois (or "red bag" in lay terms) is an obstetrical emergency.
This can be recognized by the appearance of a red, velvety membrane at the vulva before delivery of the foal.
The chorio-allantois must be ruptured immediately and delivery assisted because the foal will rapidly become hypoxic and anoxic.
Clients should be made aware of this condition because if they do not take action and await the arrival of a veterinarian, the foal will die of anoxia.
Premature placental separation is associated with placentitis (the placenta is too thick to rupture at the cervical star area), twinning, inappropriate induction methods (PGF) and unknown causes.
In many cases, the mare doesn't strain and the premature separation and failure to rupture interferes with delivery.
The foal progressively becomes hypoxic, weak, and dead.
This condition should be treated as an emergency with rupture of the membrane and delivery of the foal.