Mishnah:
The Mishnah deals with a young woman in whom blemishes were discovered, and the question underlying the entire discussion is when those blemishes developed:
If they came about after the kiddushin: it happened in the husband's domain, the kiddushin are valid, and she is entitled to her kesubah. The blemish developed under his watch, and in the language of the analogy, the husband's field was flooded, and he is the one who lost out; his bad fortune brought it about.
If she had them beforehand: he is not obligated to give her a kesubah.
Two situations in the Mishnah:
The blemishes were discovered while she was still in her father's house: it is the father's burden to bring proof that the blemishes developed only after the kiddushin. And if he cannot prove this, the assumption is that she already had the blemishes beforehand, even before the kiddushin, since they were found in his domain while she was living in her father's house. We do not assume that this happened specifically now, after the kiddushin; rather, as the Gemara puts it: here they were found and here they came to be, and the burden of proof rests on the father.
She entered the husband's domain and was married, and then the blemishes were found: it is the husband's burden to bring proof that she had the blemishes even before the kiddushin took place, so that his acquisition, meaning the kiddushin, was made in error. For since the blemishes were found in his domain, we assume that they developed in that domain.
This is the view of Rabbi Meir.
Which blemishes are we speaking about?
Hidden blemishes: blemishes concealed beneath her clothing, which cannot easily be seen in public. With these, the ruling depends on the domain in which they were found.
Visible blemishes: blemishes that are open and plain to the eye. Here the husband cannot claim that he did not know about them, for they stand before him to be seen and he married her nonetheless. Either they developed in her recently, or she had them beforehand, and in the language of the Gemara, he understood and he accepted. He cannot say afterward that he does not want her, and with visible blemishes there is no room for discussion at all.
If there was a public bathhouse in that city:
If there is a public bathhouse in that city, the same ruling applies even to hidden blemishes, those beneath her clothing. For he should have had her examined by his female relatives, who would have seen whether she had hidden blemishes or not. It was within his power to find out, and therefore he cannot claim that he did not know about them. And now, either the blemishes developed in her recently, in his domain, or she had them beforehand, and it is clear that he knew about it and did not mind.
In summary: we have learned that the ruling in the case of a young woman found to have blemishes depends on the question of in which domain they developed: blemishes that came about after the kiddushin are in the husband's domain, and she receives her kesubah; whereas blemishes that preceded the kiddushin exempt him from the kesubah. If they were found while she was still in her father's house, the burden of proof is on the father; if they were found after she entered the husband's domain, the burden of proof is on the husband. And all of this applies specifically to hidden blemishes, for with visible blemishes, and likewise in a place that has a public bathhouse, it was within his power to find out, and he cannot claim that he did not know.