REVERSAL OF CELL LAYERS IN HYDRA: A CRITICAL RE-APPRAISAL

Abstract

1. Hydra were everted by pulling their basal disc through their mouth and were then observed to determine the mechanism of regeneration. Animals that were not seen to re-turn to their original body layer orientation were fixed at time intervals from one-fourth to 144 hours and examined histologically. No evidence was found to support Roudabush's hypothesis that cell migration across the mesoglea could re-establish the normal body layer orientation. But areas damaged during everting were found which correspond with the regions of cell migration described by Roudabush. 2. The regeneration of everted hydra occurs solely as a result of re-turning by two methods: (a) slow re-turning resembling the turning of a sock when it is being removed; and (b) fast re-turning accomplished by a rapid contraction by the everted animal followed by relaxation to the re-turned or normal body layer orientation.