The new Princess line was a gown cut in one piece with a number of panels fitted and gored from shoulder to hem. It shaped the figure through seaming. The early and mid Victorian style was changing quickly into late/mid Victorian style.
There was no clear line to separate the styles; just the women who wore them as they were introduced caused everyone to take note and follow suit (or gown in this case).
(Harper’s Weekly with the very latest in 1866, housedress featuring Princess seams)