1. Medawar, P. B., J. Anat., 78, 176 (1944).
2. Allegra, F., and Giacometti, L., Acta Derm. Venereol., 48, 429 (1968).
3. Lillie, R. D., Histopathologic Technic and Practical Histochemistry (third ed.), 95 (McGraw-Hill Book Co., New York, 1965).
4. Messier, B., and Leblond, C. P., Proc. Soc. Exp. Biol. and Med., 97, 7 (1957).
5. Pease, D. C., in Histological Techniques for Electron Microscopy (Academic Press, New York, 1964).