Chullin 69 - Can a Limb be Born?
Rav said that once a fetus puts forth its foreleg, then even if it draws it back, the limb becomes prohibited. However, Rabbi Yochanan disagrees. What is the root of their disagreement? Rav says that once the limb is out of its mother's womb, it is considered to have been born. If the complete fetus is subsequently born, then its shechitah will permit the limb to be eaten. But if its mother is slaughtered, then the fetus is permitted, but the limb is forever forbidden.
Sometimes, when an animal was in difficult labor, they would cut off a piece of the fetus, to ease the pain of the animal. Since there is a necessity for this measure, it is not prohibited as cruelty to animals. If he leaves the limb inside the womb and later slaughters the mother, this limb (and the whole fetus) is permitted to be eaten. The Torah said, " Any that has a split hoof and chews its cud - among the animals ..." "Among the animals" (ba behemah) can be understood as "within an animal" - and this serves as the source for the law about a fetus.
Art: Rosa Bonheur - Doe And Fawn In A Thicket