An analysis of the spatial distribution of ciliary units in a ciliate, Euplotes minuta.

Six to eleven longitudinal ciliary rows are arrayed over the dorsal surface of the ciliate Euplotes minuta. Forty-two to 129 ciliary units are distributed among these rows. The number of rows depends on genotype, clonal age, and vegetative ancestry, while the total number of units is controlled partly by the number of rows and also by the separate action of genetic and environmental factors. 2. The pattern of distribution of units among different rows of non-dividing cells can be analysed on the basis of the percentage of the total unit complement of the cell that is found in each individual row. If the assumption is made that ciliary rows are uniformly spaced over a dorsal field whose width is independent of the number of rows, then it can be shown that units are distributed among rows according to a relatively invariant spatial pattern. The form of this pattern remains the same in the face of variation in the absolute number of rows and of units. 3. Prior to cell division new units develop anterior and posterior to old units situated within the equatorial zone of each row. About one-half of the original units are included within this zone. The cell fission line develops within this zone such that the total number of units passed to the anterior and posterior division products are about equal. 4. The pattern according to which units of different zones (proliferating and non-proliferating) are distributed among different rows has been mapped in cells that have completed the process of proliferation of units but have not yet completed cell division. The results of this mapping show that the pattern of distribution of units in the equatorial zone at the conclusion of proliferation is not the same as the overall pattern in non-dividing cells. Further analysis indicates that the geometry of proliferation can be most simply represented as a result of two superimposed processes, one of which is the recruitment of old units into the zone of proliferation, while the other is the intensity of proliferation, i.e. the number of new units formed adjacent to each old unit. Both recruitment and intensity have constant values in the central region of the dorsal field, while recruitment is higher and intensity lower near both margins. The recruitment and intensity distributions are mutually nearly reciprocal, with slight asymmetries that formally account for the more dramatic asymmetry of the pattern of non-dividing cells. 5. A dualistic hypothesis is formulated for the control of the formation of new ciliary units within ciliary rows. The position of each new unit is largely controlled locally in relation to pre-existing units, while the decision of whether or not new units will develop at all, and how many will be formed, depends on superimposed positional systems operating within the context of the entire dorsal surface.