Proportioning a Hydra

Abstract

SYNOPSIS. Pieces of hydra tissue of various sizes and shapes were cut from the body columns of adult hydra and allowed to regenerate. The proportions of the resulting animals were determined first by counting cells in the head and body, and secondly by measuring the structures directly using an ocular micrometer. Head-body proportions were found to be constant over a tenfold size range. Very small regenerates had a larger head fraction and large budding regenerates had a smaller head fraction. Extra structures developed in certain shape pieces, but total head-body tissue remained proportional. More detailed measurement of the head showed that the hypostome regulated only slightly with total size change, while the tentacle tissue varied considerably to maintain the head-body ratio. This suggested that the patterning of the hypostome and the tentacles might involve separate processes, with the latter being responsible for proportion regulation. While the body mass appeared to be determined by the proportioning mechanism, its circumference was related to the circumference of the hypostome, suggesting a causal relationship between the two organizers and the column shaping. The basal disc remained proportional to the body except in the smallest pieces. A Gierer-Meinhardt pattern formation scheme could account for the results found.