A consequence of Vershik's results on discrete-time filtrations is the existence, in continuous time, of filtrations F = (F t) t 0 which are "Brownian after zero" (that is, for each ε > 0, F ε = (F ε+t) t 0 is generated by F ε and some F ε-Brownian motion), but not generated by F 0 and any Brownian motion. Among the filtrations that are Brownian after zero, how are the truly Brownian ones characterized? An answer is given by the self-coupling criterion (ii) of Theorem 1. This criterion is always satisfied when F is immersible into the filtration of an infinite-dimensional Brownian motion.