If a man has two wives, the one beloved and the one hated, and they bear him sons, the beloved and the hated, and the firstborn son is the hated one’s—
English: “And the son shall be” — Rabbi Ammi said: a tumtum who is later opened and found to be male does not take a double portion, for the verse says “and the firstborn son shall be to the hated one” — he must be a son from the moment of existence (birth).
On the daf / in context: Parallel to the earlier ruling that such a child does not reduce others’ shares, he himself also does not receive the double portion, because he was not known as a “ben” at the critical moment.
Type of Learning
Derash requiring known male status from the time of birth for receiving the bechorah.
What Is Learned
A tumtum later revealed to be male does not himself receive the double portion of the firstborn.