If one consecrates a pregnant animal as a sacrifice and says, “The fetus of this animal is designated as a burned offering, and she herself a peace offering,” then his declaration takes effect. Since he consecrated the fetus first, his subsequent declaration applies to mother only.
However, if he said, "I designate this animal as a peace offering and its fetus as a burned offering," then both the animal and the fetus are peace offerings. His first words take effect immediately, and the mother becomes a peace offering, with everything inside her – that is the opinion of Rabbi Meir. His ruling principle as that we always “seize upon the first expression,” that is, we view the first part of his statement as decisive, and ignore any contradicting subsequent declarations.
Rabbi Yose says that we ask him about his intent. If, from the outset, he wanted the mother to become a peace offering, and the fetus – a burned offering, then we view these desires as part of the same statement. It is impossible for a human to express both desires simultaneously, but only this prevented him from doing so, thus both his intentions takes effect. Rabbi Yose's principle is that both parts of one's statement are important. Only if he changed his mind in the middle, then his subsequent changes are ineffectual.
Art: Richard Ansdell - Highland Folk - Two Lambs, a Ewe and a Fox