It is shown that a relatively weak initial shock introduced into an otherwise isentropic hollowshell implosion can cause central fuel ignition, the bulk of the fuel undergoing little increase in entropy. The imploded-state fuel configuration is analysed at the threshold of ignition, and it is found that electron heat conduction is the predominant cooling mechanism opposing fuel ignition. Approximate temperature-dependent ρR requirements are presented for the conduction-limited ignition of thin DT shells, and for the equality of electron heat conduction and α-particle transport in thermonuclear burn propagation.