1. Kerst et al.: Phys. Rev. 102, 590 (1956).
2. H. S. Snyder: Phys. Rev. 76, 1563 (1949).
3. S. I. Nikolsky and I. E. Satzevich: J. Exp. Theor. Phys. 31, 714 (1956).
4. S. Miyaki: Private communication; see also H.L. Grigorov and V.Y. Shestoperov: J. Exp. Theor. Phys. 34, 1539 (1958)
5. See the review article by K. Greisen in Progress of Cosmic Ray Physics, Vol. III, Amsterdam North Holland Publishing Co. 1956;