Zevahim 17: Levels of Impurity

The mishnah identified the person handling the offering with the status of a "tevul yom" -- one who is waiting until nightfall for the purity effected by immersing in the mikveh earlier in the day to kick in -- as one whose offering would not be valid. So the search for the source for this conclusion is undertaken by the Gemara - including a comparison to shaving the head and/or points of the face. Also, a sampling of the establishment of norms for those who might bring such a korban - in this case, a zav or one akin to a zav, with an unhealthy seminal emission (one who is a mehusar kippurim) - with a connection to the red heifer.