Complex sentence: The episode references Liz's desire to become a mother, in which she tells Carol that she is on a waiting list to adopt a child, which began in the show's first season, and continued in the third season.
Split into simpler sentences: The episode references Liz's desire to become a mother, in which she tells Carol that she is on a waiting list to adopt a child. This story first began in the show's first season, and continued in the third season.