But the placenta -- the organ that provides material to the embryo from the mother -- is actually formed from the chorion and allantois membranes of the egg.

但——为母亲的胚提供物质的器官——实际由卵子的绒毛膜和尿囊膜形成的。

Because, the main membranes of an amniotic egg -- known as the sac, the chorion, and the allantois -- are all present in animals that give birth to live young.

因为，羊膜卵的主要膜——被称为囊、绒毛膜和尿囊——都存在于生下活幼仔的物身。