by J. H. de Boer. by G. R. de Beer. by J. P. Heyboer. by M. H. de Kock. by E. R. de Ong. by H. E. de Wardener. by F. M. de Rivas. by C. W. de Kiewiet. edited by J. H. de Boer. by J. J. M. de Groot.