1. It is forbidden to castrate a young person who was already castrated in a different way, or to castrate an elderly man.
2. There is a dispute about whether a woman is commanded in procreation.
3. The Mishnah states that one is liable for tying and untying certain types of knots.
4. Rebbi Meir: Any knot that can be untied with one hand is not a knot for which one is liable for tying and untying on Shabbos.
5. The Mishnah states that there are three types of knots: those forbidden by the Torah, those forbidden by the Rabanan, and those that are permitted.
A BIT MORE
1. The former is derived from a verse, and the latter is apparent from the fact that even when someone is old it is possible for him to have children (as was the case with Rebbi Yochanan), and thus castration is forbidden.
2. The Tana Kama: She has no Mitzvah of procreation. Rebbi Yochanan ben Nuri: She has a Mitzvah of procreation. Accordingly, the Tana Kama would maintain it is permitted for a healthy woman to drink a potion that causes her not to be able to have children, while Rebbi Yochanan ben Nuri would maintain that this is prohibited.
3. It gives the example of sailor knots and camel-rider knots that are not meant to be untied (see Rashi). One transgresses the Torah prohibition for tying and untying such knots on Shabbos.
4. The Gemara is unsure about whether Rebbi Meir's main criterion for liability for tying a knot on Shabbos is that it cannot be untied with one hand, or that it is tied tightly. The difference between these two opinions is whether a tight bow that can be untied with one hand is forbidden to be tied and untied.
5. Knots that are sometimes left for one or two weeks are forbidden mid'Rabanan. Knots that are constantly tied and untied every day are permitted (according to Rashi's explanation of the Gemara).