We thank Hy-Line Poultry Farms, Johnston, Iowa, especially Dr. G.R.J. Law, for providing us with fertilized eggs to start a breeding colony of line P chickens. We are also indebted to Pharmaceutical
Manufacturers Laake Oy, Turku, Finland, for gifts of cyclophosphamide, to Kaisa Granfors, M.Sc., for editorial assistance, and toRichard A. Gatti, M.D., for reviewing the manuscript.