The form of a servant (morphēn doulou). He took the characteristic attributes (morphēn as in Phi 2:6) of a slave. His humanity was as real as his deity.
In the likeness of men (en homoiōmati anthrōpōn). It was a likeness, but a real likeness (Kennedy), no mere phantom humanity as the Docetic Gnostics held. Note the difference in tense between huparchōn (eternal existence in the morphē of God) and genomenos (second aorist middle participle of ginomai, becoming, definite entrance in time upon his humanity).