[If] the first [baby] was born dead and the second alive, [the living baby] is tahor. [If] the first [baby] was alive and the second dead, the [living baby] is tamei. R’ Meir says: [If they are in] one fetal sac, [the living baby] becomes tamei; however, [if they are in] two fetal sacs, the [living baby] is tahor.