Al manuscript.14.15. 16.17. 18. 19.AcknowledgementsThe authors are grateful to Monique De Pelsmacker, Solange Peeters, Frank Van Der Kelen and Kris Vantyghem for their excellent technical assistance and to Bert Thys for his critical advice. We also thank Dr. Albert Geerts for allowing us to use the phase-contrast microscope. Dr. Yvan Vander Heyden is thanked for his advice on the presentation of t