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Fig.1.Interactions of nucleons (full lines), pions (broken lines) and scalar-isoscalar mesons (wavy lines) present in both linear and non-linear models (a) and only in the latter (b).

= 17 cm Fig.2. Dynamical content of the effective scalar-isoscalar field in (a) nucleon-nucleon interactions, (b) the $\pi NN$ kernel and (c) the seagull term; conventions are the same as in fig.1 and the crosses in nucleon propagators indicate that they do not contain positive frequency components.

Fig.3. The effective seagull (a) as composed by the 