Cells of Oxytricha fallax possessing cytotactically inherited supernumerary dorsal bristle rows can redevelop those dorsal supernumerary rows after cystment, even though supernumerary ventral cortical structures are permanently lost through cystment. Previous work has demonstrated: (1) that cystement involves a complete dedifferentiation of all ciliary structures-all cilia, basal bodies, microtubules and fibres; and (2) that all ventral ciliary structures arise from a single ciliary primordium during excystement. These observations suggest the following conclusions. (1) The information for the redevelopment of supernumerary dorsal bristle rows during excystment is associated with some ultrastructurally unidentifiable molecular structure of the cyst cortex. (2) Cytotactic information for the development of cortical patterns is retained in at last two locations in the resting cyst; one location specifies the site of development of the ventral ciliature whereas the other specifies the location and pattern of the dorsal ciliature.