Yevamos, Chapter 8, Mishnah 2. In the previous Mishnah we learned that a petzu'a daka and a k'rus shafchah may not enter the congregation of Israel, and therefore their wives do not eat Terumah. Our Mishnah now comes to define what these blemishes actually are.
The definition of petzu'a daka:
"Eizehu petzu'a daka? Kol sheniftze'u habeitzim shelo, va'afilu achas mehen" - anyone who was injured in his reproductive organs, whether they were severed in some manner or crushed and the like. It is enough that only one of the two was damaged for him to fall under this definition.
The definition of k'rus shafchah:
"U'chrus shafchah, kol shenichras hagid" - anyone whose male organ was severed. The Mishnah, however, adds a qualification:
"V'im nishtayer mei'ha'atarah afilu k'chut hasa'arah - kasher" - even if there was a clear cut, as long as the corona at the end of the organ remains, even a thin layer of it no thicker than a hair - he is kasher.
The severing counts only when the cut was made in the body of the organ itself, and then he is a k'rus shafchah and forbidden.
Whom they are permitted to marry:
"Petzu'a daka u'chrus shafchah mutarin b'giyores u'vimshuchreres" - they may marry a convert, and likewise a maidservant who was freed, who is also included in the category of a convert. "V'einan asurin ela milavo bakahal, shene'emar: Lo yavo petzu'a daka u'chrus shafchah bikhal Hashem" - their prohibition is limited to marrying a woman who is part of the congregation of Hashem, that is, to entering into the congregation of Israel.
The Tanna'im disagree on the question of whether kahal geirim ikri kahal - whether the community of converts is also considered part of the "congregation of Hashem," so that they would be forbidden, or whether they are permitted. Our Mishnah takes a position on this issue and rules that converts are not called a "congregation" in this technical sense, and therefore they are permitted to marry a petzu'a daka and a k'rus shafchah.
This halachah has also come up in practice: a well known leader of the Old Yishuv in Jerusalem was wounded by a bullet and was a petzu'a daka, and he married a French convert. The two were a very well known couple in Jerusalem.
In summary: this Mishnah defined the two blemishes - petzu'a daka, whose testicles were injured, even one of them, and k'rus shafchah, whose organ was severed in the body of the organ, provided that not even a hair's breadth of the corona remained. We also learned that their prohibition is limited to entering the congregation of Hashem alone, and therefore they are permitted to marry a convert or a freed maidservant, since according to our Mishnah the community of converts is not called a congregation.