Leonard Nimoy was asked to allow his children to appear as extras but Nimoy refused to let his children be involved in show business. His son, Adam Nimoy, did grow up to become a television director, including a few episodes of Star Trek: The Next Generation (1987).
The first of several "parallel Earth" plots in the series, contrived to save money by avoiding the necessity for "alien" sets, costumes, and makeup.
Although all the children on the planet die when they reach puberty, Michael J. Pollard, who played Jahn, was 27 years old when this episode aired (sporting visible sideburns), and Kim Darby, who played Miri, was 19.
The outdoor scenes of this episode were filmed on the same back lot streets that were also used to create Mayberry on The Andy Griffith Show (1960), except that the streets were piled with debris and dirt to create the appearance that the town was in ruins. Several building exteriors familiar from Mayberry can be seen in those exterior shots including the courthouse, Walker's Drugstore, the Grand movie theater, Floyd's barber shop, and the Mayberry Hotel.
The girl held by Kirk as he rushes to the lab with his newly-recovered communicator is played by Bill Shatner's middle daughter Lisabeth.