Answer:
Each puppy is enclosed in a sac that is part of the placenta or afterbirth. This sac is usually broken during the birthing process and passes through the vulva after each puppy is born.
Explanation:
Puppies are born inside a thin sac, which the mother will remove, enabling them to breathe. After each puppy, your dog should pass an afterbirth (placenta), which they often eat. The placenta is the structure that provides oxygen and nutrients to the puppy while it's developing in the womb.
