When a cow births for the first time, the newborn falls under the rules of the firstborn (in the words of the Torah, “that which opened the womb”); it must be brought as a sacrifice in the Holy Temple and the priest eats of its flesh. Even in our times firstborn animals are forbidden for eating until they have become disfigured; then they are permitted. (The disfigurement can be as small as a wound to the ear.) But before the newborn is subject to the laws of the firstborn the sages required conformance to a great number of details. Thus, for example, a firstborn animal delivered by Cesarean section (in the words of the sages, “which comes out of the side”) is not subject to the sanctity of the firstborn and is not considered a firstborn, because it did not come forth through the womb and the birth canal of its mother.
The Talmudic sages indulged in polemics and debated a case in which a firstborn was covered with some sort of covering while still in its mother’s womb and then removed. (The covering would separate him from the walls of the womb.) Is he to be considered a firstborn? They went on to ask about a case in which a rat swallows the fetus while it is still in the womb and removes it from there. Do the laws of a firstborn still apply? What is the rule if a rat swallowed the fetus while it is still in the womb, removed it from there, and then returned it to the animal’s womb, from whence it made its way out on its own? Was this second exit considered a natural birthing, and so the calf is considered a firstborn? And what is the rule if the dairy farmer placed the wombs of two cows next to each other, so that one cow births its calf directly into the womb of a second cow, who then births the calf to the outside world? Is the calf considered a firstborn? (These questions are not answered in the Talmud.)
(Babylonian Talmud, Tractate Chullin 70a)