The issue of isotropic-anisotropic equilibria contributions to structure formation of phase inversion (PI) membranes of semiflexible polymers such as cellulose derivatives is addressed. Two types of ternary phase diagrams pertinent to the PI of semiflexible polymer dopes are suggested, and modes of phase separation that control structure formation are described. Alternative interpretations for certain earlier structural observations in cellulose acetate (CA) membranes are proposed. Discussions of the lyotropic polymer gelation modes and the effect of glass formation on the progress of PI structural transformations are included.