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The patterning and organization of new structure in eukaryotic cells can, in certain cases, be controlled by existing structure. Sonneborn illustrated this in Paramecium two ways. (1) Cells with a doublet phenotype (individual cells with almost two complete sets of cortical structures) develop true to type and are inherited, sexually and asexually, independent of changes in nuclear genotype or fluid cytoplasm (Sonneborn, 1963). (2) Inverted rows of cilia and basal bodies maintain their inverted polarity during morphogenesis and cell division, again independent of changes in nuclear genotype or fluid cytoplasm (Beisson & Sonneborn, 1965). These observations led to formulation of the concept of ‘cytotaxis’ – the organization of new structure under the influence of preexisting structure (i.e. the molecular architecture of the cell which includes visible structure) (Sonneborn, 1964,1975). Cytotaxis thus represents an informational system for pattern development and regulation which is not under direct nuclear control. Several examples of cytotaxis have been demonstrated subsequently in other cell types (Grimes, 1973c, 1976; Grimes & Adler, 1978; Grimes & L’Hernault, 1979; Hammersmith, 1976b); Ng & Frankel, 1977).

Heteropolar doublets of the hypotrichous ciliate Oxytricha fallax can encyst, as can monster cells containing multiple copies of at least part of the ciliature. The resulting cysts excyst as heteropolar doublets or monster cells, respectively. Transmission electron microscopy of resting cysts derived from monster cells and from singlets, however, indicates a complete loss of the ciliature - all kinetosomes, ciliary axonemes, and microtubules. Thus, the determination of each ciliature of a monster cell cannot be attributed to the persistence of any visible part of the ciliature. Transmission electron microscopy of monster and singlet cysts suggests that the information for a ciliature may be related to the system of cyst wall grooves and possibly to differentiations in the inner cell membrane which separates the cyst cytoplasm from the cyst wall. The redevelopment of heteropolar doublets upon excystment indicates that the anterior-posterior polarity is independently determined for each ciliature and is maintained through cystment. This polarity is not, however, reflected in a recognizable anterior-posterior polarity of any visible structure of the resting cyst. The number of adoral zones of membranelles (AZMs) may increase or decrease in some excysting monster cells compared with that observed in the encysting cells. Possible explanations for these changes are offered in the Discussion.

Singlet cells of Oxytricha fallax possessing supernumerary rows of dorsally located marginal cirri were encysted, excysted, and observed for retention of the supernumerary rows. Without exception, cells lost the supernumerary rows as a result of cystment processes. (Encystment previously has been shown to result in the total resorption of ciliature.) In contrast, supernumerary dorsally located marginal cirral rows develop true-to-type during prefission morphogenesis and thus are inherited from cell to cell. These (and other) observations suggest that at least two levels of cytotactic control of cell patterning are operative on the ciliate cortex; one is dependent upon visible ciliature whereas the other is dependent upon an as yet ultrastructurally unidentifiable molecular architecture.