We have already learned (see, for example, daf 48) that kodashei kodashim – the holiest of sacrifices – must be slaughtered and prepared in the northern part of the Temple courtyard, while kodashim kalim – sacrifices that are on a lower level of holiness – can be slaughtered and prepared anywhere in the Temple courtyard. In the Mishnah on today’sdaf (=page), Rabbi Yossi teaches that if kodashei kodashim were slaughtered on the altar itself, that would be permissible, while Rabbi Yossi b’Rabbi Yehudah rules that only the northern half of the altar would be acceptable; the southern part is not considered “north” and only kodashim kalim could be slaughtered there.
In explanation of these two opinions, the Gemara refers to the passage in Sefer Shemot (20:20) that teaches that the earthen altar is the place where olot — burnt-offerings (kodashei kodashim) — as well as shelamim — peace-offerings (kodasim kalim) — are sacrificed. Rabbi Yossi understands the pasuk (=verse) to mean that the altar is a place where both types of sacrifice can be slaughtered; Rabbi Yossi b’Rabbi Yehudah understands the pasuk to mean that half the altar is available for olot and the other half is available for shelamim.
From the Mishnah it sounds as if sacrifices that were slaughtered on the altar were considered valid ex post facto, but that the preparation really should not have been done there. Tosafot point out that based on the Torah passage itself, it appears that slaughtering the animal on the top of the altar is appropriate even le-khathilah — in the first instance. They argue that the Mishnah reflects the common practice to avoid doing that because of the concern lest the altar be defiled by the animal, for example, if the animal defecates on it while being prepared for slaughter.