From the Department of Orthodontics, Eastman Dental Center, Rochester, N.Y. The research was supported in part by Grant DE-01837, National Institute of Dental Research, NIH, USPHS.

Read before the Angle Society of Orthodontia, Boston, Massachusetts, October, 1965.

Adapted from the Grieve Memorial Lecture, Canadian Dental Association, June, 1964 and read in part before the Midwestern Component of the Angle Society, Fort Lauderdale, January, 1965.

* From the Child Study Clinic, University of Oregon Dental School. This investigation was supported (in part) by Grant HD 00157-05 from the Institute of Child Health and Human Development, National Institutes of Health, U.S.P.H.S.

* This study was supported in part by U.S. Public Health Service Grants DE-19,471 and D-1468 from the National Institute of Dental Research, National Institutes of Health, Bethesda, Maryland.

From the Department of Orthodontics, University of Illinois.

From the Orthodontic Department of the University of Manitoba and the Winnipeg Children's Hospital, Winnipeg, Manitoba.

This investigation was supported by Grant MA-1681 from the Medical Research Council, National Research, Ottawa, Canada.

This investigation was supported by Grant D-1829, N.I.D.R., U.S.P.H.S.

This study was financed in part by U.S. Public Health Service Grant Number FR-05305-01.