MARY ANN UMSTEADUmstead-MackellMr. and Mrs. Edwin A. Umstead of Fountainville announce the engagement of their daughter Mary Ann to Dr. Thomas E. Mackell of Perdlyn.Dr. Mackell is the son of Dr. and Mrs. James V. Mackell of Huntingdon Valley and Stone Harbor, N.J.Miss Umstead graduated from Allentown Hospital School of Nursing and will attend Thomas Jefferson University. She is a registered nurse employed at Doyleatown Hospital.Dr. Mackell is a graduate of Georgetown University and Jefferson Medical College. He is an orthopaedic surgeon on the staff at Doylestown Hospital.A Dec. 4 wedding is planned.