Along with another uncapped player - Bath back Jack Cuthbert - Jon and Rob have been included in the 34-man squad for next week's training camp in St Andrews.
Scotland have won five of their last six games and begin the Six Nations against France in Paris on February 5.
Centre Graeme Morrison is unavailable due to a knee injury sustained playing for Glasgow against Edinburgh on December 27, while Chris Cusiter (Glasgow), Phil Godman (Edinburgh), Alasdair Dickinson and Jim Hamilton (both Gloucester) were also not considered by coach Andy Robinson due to injury.
Johnnie Beattie and Simon Danielli return to the squad after missing the autumn Tests, which featured defeat to New Zealand and wins over world champions South Africa and Samoa.
Robinson said: "There's always excitement in the air at this time of year as we look forward to the RBS Six Nations Championship.
"Selection reflects both current form and the performances of a number of players during our matches in the autumn.
"I've been impressed by Robert Harley since I first saw him play in a pro-team back-up game some seasons back.
"He has performed consistently for Glasgow this season in a position where we have strength and there is considerable competition for a starting place.
"Jon Welsh is another who has benefited from raising his game in the face of intense competition and has a good work ethic.
Scotland squad for pre-Six Nations Championship training camp at St Andrews, beginning on Monday:
Backs: Joe Ansbro (Northampton Saints), Mike Blair (Edinburgh), Jack Cuthbert (Bath), Simon Danielli (Ulster), Nick De Luca (Edinburgh), Max Evans (Glasgow Warriors), Alex Grove (Worcester Warriors), Ruaridh Jackson (Glasgow Warriors), Greig Laidlaw (Edinburgh), Rory Lamont (Toulon), Sean Lamont (Scarlets), Rory Lawson (Gloucester), Dan Parks (Cardiff Blues), Chris Paterson (Edinburgh), Hugo Southwell (Stade Français) and Nikki Walker (Ospreys).
Forwards: John Barclay, Johnnie Beattie (both Glasgow Warriors), Kelly Brown (Saracens), Geoff Cross, Ross Ford (both Edinburgh), Richie Gray, Dougie Hall, Robert Harley (all Glasgow Warriors), Nathan Hines (Leinster), Allan Jacobsen (Edinburgh), Alastair Kellock (Glasgow Warriors), Scott Lawson (Gloucester), Moray Low (Glasgow Warriors), Scott MacLeod (Edinburgh), Euan Murray (Newcastle Falcons), Ross Rennie (Edinburgh) and Richie Vernon and Jon Welsh (both Glasgow Warriors).