Experimental evidence relating to the early phase of the spark ignition process is reviewed. This suggests that the evolution of the spark kernel after the initial shockwave formation is governed by well defined fluid mechanical structures. In the case of an axial spark this takes the form of a toroidal vortex pair which, depending on the discharge conditions, can be either a laminar or turbulent structure, with a rapid transitions between the two states. Plasma jet and surface discharge igniters on the other hand produce an axially propagating spherical turbulent kernel. It is proposed that igniters can be classifed into two major groups on the basis of their underlying fluid mechanical structures, either radially or axially propagating mixing elements. A mathematical model of the turbulent toroidal mixing element is developed and compared with some of the available experimental data on kernel growth rates. The consequences of the internal vorticity of the kernels for ignition in turbulent mixtures are examined and some of the relevant published data is re-interpreted in terms of the turbulent mixing model.