Bekhorot בכורות

The firstborn of clean cattle is holy from birth and is offered; if it has a blemish it stays with the priest and is eaten as ordinary meat. Hence the long lists of blemishes—in animals and, separately, in priests, whom a blemish bars from service.

A chapter of its own deals with the redemption of a firstborn son: five shekels to a priest on the thirtieth day. It is the one commandment of the tractate still performed, and the rite of pidyon ha-ben comes straight from here. At the end is the tithe of cattle, counted out by passing the animals under the rod.

Order: Kodashim. There is Gemara in the Babylonian Talmud.