Chesterfield have signed former Aston Villa defender Chris Herd on a deal until January, while West Bromwich Albion defender Jack Fitzwater has joined on a 28-day loan.
Herd, 26, played 42 times for Villa and spent time on loan at Bolton Wanderers and Wigan Athletic last season.
Fitzwater, 17, has been training with the League One side and could play against Colchester on Saturday.
"I think it's a great opportunity with this team," Herd told the club website.
The Australia international has not joined in time to be eligible for the game against the U's.
Boss Dean Saunders added: "You can see from his career the pedigree that he's got and we'll be looking to get him into tip-top condition so that he's able to show that again."