The youngest is 20-year-old Glasgow Warriors open-side flanker John Barclay, whose club-mate, wing Thom Evans, 22, also gets a chance to stake a claim.
Former Border Reivers pair Calum MacRae, 27, and Nick De Luca, 23, feature among the backs, while Edinburgh's 23-year-old prop Alasdair Dickinson also got the nod from Hadden who will name his definitive World Cup squad in August.
Hadden said: "This week most of the players are embarking on a five-week fitness, strength and conditioning camp, under the direction of our fitness coach, Mark Bitcon.
"Phase two will see the emphasis very much on rugby and will lead into our Bank of Scotland Corporate August Tests against Ireland and South Africa.
"Our intention is to make this the best prepared and fittest Scotland squad ever for a Rugby World Cup."
Hadden is also keeping the door open for Ulster wing Simon Danielli.
Hadden added: "The groin injury he sustained against Ireland A on Saturday does not appear to be as bad as first thought and he will be subject to further assessment when he returns from holiday."
Scotland's World Cup training squad:
M Blair (Edinburgh), C Cusiter (Border Reivers), N De Luca (Border Reivers), R Dewey (Edinburgh), M Di Rollo (Edinburgh), T Evans (Glasgow Warriors), A Henderson (Glasgow Warriors), R Lamont (Glasgow Warriors), S Lamont (Northampton), R Lawson (Gloucester), C MacRae (Border Reivers), G Morrison (Glasgow Warriors), D Parks (Glasgow Warriors), C Paterson (Edinburgh), G Ross (Castres), H Southwell (Edinburgh), N Walker (Ospreys), Simon Webster (Edinburgh), J Barclay (Glasgow Warriors), K Brown (Border Reivers), D Callam (Edinburgh), A Dickinson (Edinburgh), R Ford (Border Reivers), D Hall (Edinburgh), J Hamilton (Leicester), N Hines (Perpignan), A Hogg (Edinburgh), A Jacobsen (Edinburgh), A Kellock (Glasgow Warriors), G Kerr (Border Reivers), S Lawson (Glasgow Warriors), S MacLeod (Llanelli Scarlets), E Murray (Glasgow Warriors), S Murray (Edinburgh), C Smith (Edinburgh), A Strokosch (Edinburgh), S Taylor (Edinburgh), J White (Sale Sharks).