Acc to me we dnt need any external agent for mesomeric effect also. As it only means the transfer of pi electrons from a multiple bond to an atom or from a multiple bond to a single covalent bond or u can also say tht it is transfer of lone pair of p-electrons from an atom to the adjacent single covalent bond.

And +M effect is only due to the transfer of  electron pair away from the atom and  -M is due to transference of electron pair towards the atom, so there is no relation with an external agent in mesomeric effect. Hence resonance and mesomeric effect is same. i hope nw u got ur answer. thanx...