Complex sentence: Medary remained with Frank Miles Day until 1894, when he opened his own firm in Philadelphia, Field & Medary; that firm would eventually become Zantzinger, Borie & Medary in 1910.
Split into simpler sentences: Medary remained with Frank Miles Day until 1894, when he founded his own firm in Philadelphia, Field & Medary. That firm would become Zantzinger, Borie & Medary in 1910.