QUESTION: The Gemara concludes its list of the Sheniyos l'Arayos with the six additional Sheniyos of Rebbi Chiya. His first four Sheniyos are the daughter of the third generation through one's son, the daughter of the third generation through one's daughter, the daughter of the third generation through one's wife's son, and the daughter of the third generation through one's wife's daughter.

RASHI (DH Shlishi and DH v'she'b'Bito) explains that these relations refer to the daughter of the son of one's son, the daughter of the son of one's daughter, the daughter of the son of one's wife's son, and the daughter of the son of one's wife's daughter.

According to Rashi's explanation, why does Rebbi Chiya discuss only the daughter of the son of one's son or daughter, and not the daughter of the daughter of one's son or daughter? Similarly, why does he mention only the daughter of the son of one's wife's son or daughter, but not the daughter of the daughter of one's wife's son or daughter? According to Rashi, Rebbi Chiya mentions only cases in which there is a woman at the end (obviously, since he is discussing a prohibited woman) and at the beginning, but not when there is a woman in the middle generation. Why does Rashi exclude all other cases of equivalent relatives from Rebbi Chiya's list of Sheniyos? (Indeed, the RAMBAM in Perush ha'Mishnayos and the BARTENURA include such cases in their list of the Sheniyos of Rebbi Chiya.)

ANSWER: RAV ELAZAR MOSHE HA'LEVI HOROWITZ answers that the Gemara later questions whether or not the Sheniyos of Rebbi Chiya have a "Hefsek" (they extend to further generations). In the case of the daughter of one's daughter's daughter, in which the first, second, and third generations are all women, it is obvious that the prohibition extends through the generations because both the first generation (one's daughter) and the second generation (his daughter's daughter) are Asur mid'Oraisa. Whenever there is any generation of a particular relation which is Asur mid'Oraisa, that relation is Asur throughout all of the generations (as Rashi explains on 21a, DH v'Eshes; see Chart there). Accordingly, if Rebbi Chiya refers to the daughter of one's daughter's daughter, the Gemara would not question whether it has a "Hefsek" or not.

The question applies only in the case of the daughter of the son of one's daughter. In that case, although the first generation (his daughter) is Asur mid'Oraisa, the son in the middle interrupts the Isur; there is no case of an Ervah with a son in a middle generation, following a daughter, which is Asur mid'Oraisa. Hence, the Gemara asks whether or not the prohibition of the daughter of the son of one's daughter continues for ensuing generations.