The Gemara relates that all of the Talmidim of Rebbi Akiva died during one period from Pesach to Shavuos. The world became desolate ("Shamem") of Torah until Rebbi Akiva reared five new Talmidim -- Rebbi Meir, Rebbi Yehudah, Rebbi Yosi, Rebbi Shimon, and Rebbi Elazar ben Shamua -- who sustained the world with their Torah-learning.
When the Gemara in Sanhedrin (14a) describes the Mesirus Nefesh of Rebbi Yehudah ben Bava, it lists the same five Tana'im as those for whom Rebbi Yehudah ben Bava risked his life to grant them Semichah before he was killed by the Romans. Rav Avya there adds the name of Rebbi Nechemyah to the list. In the Midrash's account of the story of the Talmidim of Rebbi Akiva, Rebbi Nechemyah's name is included in the list of Rebbi Akiva's Talmidim, while Rebbi Elazar ben Shamua's name is omitted (Koheles Rabah 11:6, Bereishis Rabah 61:3). (The Midrash also adds two other Talmidim who apparently were secondary to the first five.)
RAV YITZCHAK ISAAC CHAVER (in a manuscript quoted by Rav Moshe Shapiro shlit'a) finds an allusion to these five Talmidim in a verse in Parshas Balak. When Bil'am sought to curse the Jewish people, he took Balak to "Rosh ha'Pe'or that overlooks the plains of the Yeshimon" (Bamidbar 23:28). Rashi explains that Bil'am brought Balak to that place because it was there that the Jewish people were destined to be punished for their sin at Pe'or (Bamidbar 25:3). Rav Yitzchak Isaac Chaver explains that the word "Yeshimon" alludes to another time at which the Jewish people would suffer -- the time at which the Talmidim of Rebbi Akiva died. At that time, the only ones who would survive to perpetuate the Torah in the world would be the five Talmidim whose names form the abbreviation, "Yeshimon" -- Rebbi Yehudah, Rebbi Shimon, Rebbi Yosi, Rebbi Meir, and Rebbi Nechemyah. ("Yeshimon" is spelled with two Yuds and no Vav. The word "Yeshimon" itself is related to the word, "Shamem," "desolate," which the Gemara here uses to describe the desolate state of the world until these Talmidim learned Torah from Rebbi Akiva.)
OPINIONS: The Gemara teaches that before a man departs on a trip, he must be with his wife ("Lifkod Es Ishto"). The Gemara adds that this obligation applies even when she is "Samuch la'Vestah," near the time at which she expects to become a Nidah. One who fulfills this obligation is assured that "... you will visit your home and you will not sin" (Iyov 5:24).
The Gemara in Nidah (63b) teaches that a man must separate from his wife when she is "Samuch la'Vestah." Why, then, is he obligated to be with his wife before he departs on a trip when she is "Samuch la'Vestah"?
(a) The RASHBA and RITVA explain that the prohibition to be with one's wife when she is "Samuch la'Vestah" is mid'Rabanan, and the obligation to be with her when he departs on a trip is also mid'Rabanan. The Rabanan have the right to override their own enactment where they see fit to do so. This appears to be the opinion of RASHI here as well.
(b) The RITVA in Shevuos (18b) suggests that even if the prohibition to be with one's wife when she is "Samuch la'Vestah" is mid'Oraisa, the Torah itself teaches that a man is permitted to be with his wife immediately before he departs on a trip and the prohibition of "Samuch la'Vestah" does not apply. The verse concludes with the assurance, "And you will not sin"; since he fulfills his obligation to be with his wife for the purpose of a Mitzvah, he is assured that his wife will not become a Nidah at that moment even though she is "Samuch la'Vestah."
(c) The NIMUKEI YOSEF here and the RITVA (in the name of the RAMAH) in Shevuos explain that when the Gemara says that a man must be with his wife before he departs on a trip even when she is "Samuch la'Vestah," it does not refer to the same state of "Samuch la'Vestah" as the Gemara in Nidah. The Gemara here means "close to the time when she will go to the Mikvah," and not close to the time when she plans to experience her menstrual flow. "Samuch la'Vestah" here refers to a woman who is already a Nidah at the time her husband plans to depart. The Gemara teaches that if she is close to the time of her Tevilah when he plans to depart (within twelve hours), he should wait for her to become Tahor so that he may be with her before he departs.
According to this explanation, the following words of the Gemara are understood. The Gemara says that one who departs on a trip for the purpose of a Mitzvah is not obligated to be with his wife because "he might become distracted [from his trip for a Mitzvah]." According to the Nimukei Yosef's explanation, this means that he should not delay his trip in order to wait for his wife's Tevilah, because by delaying he might lose the opportunity to do the Mitzvah.
(d) Other Rishonim explain that the requirement of "Lifkod Es Ishto" does not refer to marital relations at all. Rather, it refers to other signs of affection. TOSFOS (DH Chayav) in the name of RABEINU TAM explains that the Gemara means that before a man departs on a trip, he must go home and talk affectionately to his wife and formally take leave of her.
The Gemara, however, says that this should be done even "Samuch la'Vestah," even when it is close to the time she expects to become a Nidah. If "Lifkod Es Ishto" means merely that he must take leave of her, why may he not do so even when she is an actual Nidah?
The RITVA here explains that when a man departs on a trip, his wife's desire for him is stronger than usual. Speaking affectionately to her before he leaves may lead to intimacy, and thus he may not speak affectionately to her if she is a Nidah but only if she is "Samuch la'Vestah."
TOSFOS explains that the Girsa of the Gemara, "even if his wife is Samuch la'Vestah," is incorrect and should read instead, "even if his wife is a Nidah." The Gemara teaches that he indeed should take leave of her even if she is a Nidah.
(e) The RASHBA (Toras ha'Bayis 7:2) explains that when the Gemara says that one should "visit" ("Lifkod") his wife before he departs on a trip, it refers to "Divrei Hergel," showing strong signs of affection (such as Chibuk v'Nishuk) which are normally prohibited when his wife is "Samuch la'Vestah." Since such strong signs of affection are prohibited only mid'Rabanan, the Rabanan permitted them when he is going on a trip. (Of course, when his wife is an actual Nidah all such signs of affection are prohibited.)
HALACHAH: The SHULCHAN ARUCH (YD 184:10) rules in accordance with the Gemara. The REMA cites the first two opinions ((a) and (b) above) which permit relations with one's wife even when she is "Samuch la'Vestah." The Rema adds, however, that "ha'Machmir Tavo Alav Berachah" -- it is better to be stringent and to refrain from relations, and instead to show signs of affection as Tosfos (d) writes. The SHACH there (#27) adds that even Chibuk v'Nishuk is permitted, as the Rashba (e) writes. The Rema also cites the opinion of the Nimukei Yosef (c) that if his wife is a Nidah but will immerse herself within twelve hours from the time he wants to depart, he should delay his trip in order to be with her after the Tevilah.
QUESTION: The Gemara teaches that a man who marries "Bas Achoso" (the daughter of his sister) is rewarded with the assurance, "Then you will call, and Hash-m will answer; you will cry out, and He will say, 'Here I am'" (Yeshayah 58:9).
Why does the Gemara apply this assurance specifically to one who marries his sister's daughter, and not his brother's daughter ("Bas Achiv"), when both are his nieces and are related to him in the same way?
(a) The RAMBAM (Hilchos Isurei Bi'ah 2:14) writes that one who marries his sister's daughter is praiseworthy, and the same applies to one who marries his brother's daughter. Why does the Gemara specify only "Bas Achoso"? TOSFOS quotes the RASHBAM who explains that the Gemara mentions "Bas Achoso" simply because it is more common for one to marry the daughter of his sister, since one's sister tends to entreat him to marry her daughter more than his brother does. Since it is more common for such a request to occur, the Gemara says that one who concedes to that request will be blessed.
The CHIDA (Birkei Yosef EH 2:5) quotes a similar explanation in the name of RABEINU TUDRUS (a Rishon who wrote a commentary on Yevamos). He says that the Gemara refers to marrying his sister's daughter because it wants to encourage a man to help his sister when she is poor (such as when she is a widow). One's brother, in contrast, earns money himself and does not need to have his daughter marry her uncle.
(b) RASHI suggests that it indeed is preferable to marry the daughter of one's sister because it is natural for a man to love his sister more than he loves his brother, and thus the marriage with her daughter will be more successful.
TOSFOS mentions a similar idea in the name of RABEINU TAM who points out that children tend to resemble the brother of their mother (Bava Basra 110a). Therefore, a man's sister's daughter is likely to be more similar to her uncle, and the marriage will fare better.
(c) TOSFOS later (99a, DH Safek) quotes the RIVAN who says that not only is it preferable for one to marry the daughter of his sister, but it is improper to marry the daughter of his brother. When one marries the daughter of his brother, he risks annulling the Mitzvah of Yibum; if he dies childless, his brother's daughter falls to her father for Yibum, and he obviously cannot do Yibum with her.
HALACHAH: The REMA (EH 2:6 and 15:25) cites both opinions with regard to whether one is encouraged to marry specifically "Bas Achoso" or also to marry "Bas Achiv."
However, RABEINU YEHUDAH HE'CHASID writes in his will (#22) that a person should not marry the daughter of his brother or sister. The NODA B'YEHUDAH (EH 2:79) and others wonder why Rabeinu Yehudah he'Chasid says such a thing which contradicts the Gemara and all of the Poskim, who clearly state that marrying one's niece is a commendable act.
RAV REUVEN MARGOLIYOS (in "Mekor Chesed," his footnotes to Sefer Chasidim) quotes the EVEN HA'ROSHAH (#31) who explains that the Sefer Chasidim is consistent with his reasoning elsewhere (#488), where he writes that a "Chasid," a righteous person, who marries the daughter of his sister is praiseworthy. If, however, the uncle is not righteous and he seeks to marry his niece only because he is physically attracted to her, he should not marry her.
The Even ha'Roshah adds that, biologically, a significantly higher incidence of genetic defects is present in births produced from unions between close relatives. Therefore, when one marries his sister's daughter with pure intentions, Hash-m will protect him and ensure that his offspring will be healthy. If, however, he marries her with impure intentions, his fate will be left to the natural course and he will face a higher risk of genetic defects in his offspring. Rabeinu Yehudah he'Chasid advises against marrying one's niece because nowadays it is rare for a person to act with such pure intentions.
This may be the intention of the Gemara when it applies the verse, "Then you will call, and Hash-m will answer," to one who marries his niece. When one marries his niece, it indeed is necessary for him to "call out to Hash-m" and pray that He protect them from having a child with defects. If he married her with pure intentions, "Hash-m will answer" his prayer.