R. Yoshiyah says: The first was stated first, and "firsts" are not expounded. Why is the second stated? A clean animal confers tumah (uncleanliness) by being carried, and an unclean animal confers tumah by being carried. If you have learned about a clean animal that it is forbidden to cook its flesh in its (mother's) milk, I might think that the same holds true for an unclean animal. It is, therefore, written "in the milk of its mother," and not in the milk of an unclean animal. The third: "in the milk of its mother," but not in human milk.