  In his first November 4, 1915 paper Einstein wrote the Lagrangian form of his
field equations. In the fourth November 25, 1915 paper, Einstein added a trace
term of the energy-momentum tensor on the right-hand side of the generally
covariant field equations. The main purpose of the present work is to show that
in November 4, 1915, Einstein had already explored much of the main ingredients
that were required for the formulation of the final form of the field equations
of November 25, 1915. The present work suggests that the idea of adding the
second-term on the right-hand side of the field equation might have originated
in reconsideration of the November 4, 1915 field equations. In this regard, the
final form of Einstein's field equations with the trace term may be linked with
his work of November 4, 1915. The interesting history of the derivation of the
final form of the field equations is inspired by the exchange of letters
between Einstein and Paul Ehrenfest in winter 1916 and by Einstein's 1916
derivation of the November 25, 1915 field equations.
