1. For a discussion of the particle aspect in quantum field theory compareR. Haag:Dan. Mat. Fys. Medd.,29, 12 (1955), especially chap. I, no. 1.
2. CompareE. Fermi andC. N. Yang:Phys. Rev.,76, 1739 (1949). In this paper the hypothesis that π-mesons may be composite particles is discussed.
3. In this connection an interesting example is Gürsey’s model of a theory of elementary particles (F. Gürsey: to be published). There, following a suggestion of Heisenberg, all particles appear as composite and no elementary particles correspond to the basic fields.
4. This possibility was first mentioned byN. Bogoljubov: unpublished lecture notes.
5. TheS-matrix of the model considered is causal according to the definition given inH. Lehmann, K. Symanzik andW. Zimmermann:Nuovo Cimento,6, 319 (1957).