  It is shown that the endpoint of the first order transition line which merges
into a crossover regime in the phase diagram of the Nambu--Jona-Lasinio model,
extended to include the six-quark 't Hooft and eight-quark interaction
Lagrangians, is pushed towards vanishing chemical potential and higher
temperatures with increasing strength of the OZI-violating eight-quark
interactions. We clarify the connection between the location of the endpoint in
the phase diagram and the mechanism of chiral symmetry breaking at the quark
level. We show how the 8q interactions affect the number of effective quark
degrees of freedom. We are able to obtain the correct asymptotics for this
number at large temperatures by using the Pauli-Villars regularization.
