There are two major foci for the discussion of pru u'rvu in the gemara in Yevamot (61b-65b). The first is how one fulfills this commandment, while the second one is who has to fulfill it. For methodological reasons, we will begin with the second one (although the order in which the gemara presents them is an interesting issue in and of itself).

The Mishna on Yevamot 65b states that only a man is commanded to be fruitful and multiply, while women, who obviously have to play a role, do not fulfill any particular commandment by doing so. However, based on the verse that serves as the source for this law, the issue is not so clear. Rav Yochanan ben Broka counters that since Bereishit 1:18 says "and the Lord blessed them," it must be that the commandment applies to both men and women, since the initial blessing was given to both Adam and Chava. However, the gemara counters that the phrasing of the blessing is the key; having children is supposed to be done for the purpose of conquering the land. Since the task of conquering is one that traditionally falls on the shoulders of males, thus it is learned that only males are commanded to be fruitful and multiply. Furthermore, since the word "v'kivshuha" is written with a letter missing, the gemara learns out that women are excluded from this commandment.

A bit further in the gemara, Rav Yoseif tries to bring a proof from another verse elsewhere in the Torah. Yaakov is told to be fruitful and multiply as well, but instead of being told "pru u'rvu," in the plural, he is commanded "preh u'rveh," in the singular, implying that only he, and not his wives, were commanded to bear children.

The second major topic is how one fulfills pru u'rvu. As we noted last week, there are some reasons for this commandment that seem to imply that having even one child serves as a fulfillment. On the other hand, there are those who claim that since the goal is the population of the world, one must have at least one son and one daughter, since only with both males and females can the world be populated.

The main debate on this point is in Yevamot 61b. There, Beit Shammai rule that one fulfills this commandment by having two sons, while Beit Hillel claim that one must have both a son and a daughter. The gemara notes that Beit Shammai base their view on the case of Moshe Rabbeinu, who had only two sons, while Beit Hillel use creation as a model, that both male and female were created at the beginning of time. Beit Hillel reject the use of Moshe as a proof, since he separated from his wife in order to always be pure and ready to receive communication from Hashem (although Beit Shammai could argue that Moshe did not separate until he had fulfilled this commandment, and thus two sons must serve as a fulfillment).

Two statements attributed to Rav Natan are then cited in the gemara. In the first statement, he writes that Beit Shammai actually require a person to have two sons and two daughters, based on the verse concerning the birth of Hevel, son of Adam and Chava. In the second statement, Rav Natan claims that Beit Shammai require a person to have both a son and a daughter, while Beit Hillel require either a son or a daughter, since the world is settled more with each person born, and thus it does not matter if the child is male or female.

The real focus of how one fulfills this commandment can be seen in what seems to be a secondary issue. If a person has a child who then passes away, has he fulfilled the commandment of being fruitful and multiplying? If the goal is to have a certain number of souls come into existence so that Moshiach will come, then it would seem that he has done his job. On the other hand, if populating the Earth is the aim, then presumably a child who passes away does not contribute to a person's fulfillment of this commandment. The major poskim follow the line of thinking that we are concerned with populating the Earth, and thus we rule that a person must have a son and a daughter, and that if those children die without having children of their own, then their father has not fulfilled his obligation. As a result, a person who gives birth to children who are sterile has also not discharged his obligation, since these children will not contribute to the ongoing population of the world.

This series of laws is somewhat disturbing. If the goal is to populate the Earth, then why should a person be held responsible if his children are unable to do the same? He has done his part, and for some reason his children will be unable to do theirs - this should be two separate issues! Perhaps the reason for this goes back to something mentioned last week. The reason that we are supposed to dominate the Earth is not a reason in and of itself for us to have children. Rather, we are told to populate the Earth due to the fact that we are created in the image of Hashem. Thus, the sheer numbers are not important as much as the fact that there has to be an ongoing chain of people walking on this planet, testifying through their existence to the glory of Hashem. As such, a person is commanded not only to have children, but also to ensure that there will be many future generations that can do the work and will of Hashem. For this reason, as Ritva notes, there is no person of the opinion that having only daughters fulfills this obligation - since women are not commanded in pru u'rvu, a person does not ensure the perpetuation of the species in the halachic sense by having only girls (theoretically, a girl has no halachic incentive to have children, while a boy does have such an incentive and thus having boys more or less ensures that there will be future generations).