ABSTRACT
The regulation of hydra into hypostomal and non-hypostomal regions is considered as a model for the regulation of a linear pattern into two compartments. Regulation of such a system requires graded differences and a ‘principle of limited realization’.
The hypostome is the dominant region in hydra and the time for its determination in isolated pieces depends upon the original position of the piece along the linear axis. The time increases in a disto-proximal direction from 4 h to more than 20 h.
The determination of the hypostome is the first detectable event in distal regeneration and is probably the time-determining step in this process. Work on other hydroids is reinterpreted in these terms.
The time for hypostome determination in proximal regions is significantly affected by the presence of more proximal regions, while in distal regions the time appears independent of the presence of more proximal regions.
