Union County Prosecutor Theodore Romankow says 34-year-old Michael Bell of Plainfield was arrested late Saturday night near Penn Station in Newark, where he was planning to take a bus to an undisclosed site.
Bell was being held Sunday without bail, and it's not known if he has a lawyer.
Bell faces an aggravated assault charge in the matter, and Romankow says additional charges are expected.
Plainfield police responded to a Lee Place home around 10:35 p.m. Friday and found the infant, who was suffering from fractures and internal bleeding.
The child remained hospitalized Sunday in grave condition.