1. Laboratory of Histology and Pathology, National Institute of Dental Research, National Institutes of Health, Department of Health, Education, and Welfare, U.S. Public Health Service, Bethesda, Md
2. Laboratory of Histology and Pathology, National Institute, of Dental Research, National Institutes of Health, Department of Health, Education, and Welfare, U.S. Public Health Service, Bethesda, Md