492 J. S. HUXLEY AND G. R. DB BEER 



often takes place in one or more places on the tail (unpublished 

 observations, J.S.H.). Dedifferentiation is not, however, 

 followed by resorption in this case, at least before death. 



Mtiller (1913, 1914) has conducted an elaborate series of 

 experiments with various species of Hydroids. He finds that 

 dedifferentiation and resorption may occur not only in 

 hydranths but also in gonophores and in portions of hydro- 

 caulus. There is a delicately balanced equilibrium between 

 various parts of a system ; in a compound system, whether 

 gonophore, hydranth, or stem shall be resorbed depends 

 (a) on the relative sizes and (&) on the ages of the sub-systems 

 (cf. Perophora, Huxley, 1921 h). Wounds will induce gono- 

 phores to dedifferentiate and be resorbed. 



The quantitative action of poisons in accelerating dedifferen- 

 tiation and resorption, and the fact that in severed zooids 

 dedifferentiation may proceed independently of resorption, are 

 points on which we would like to lay stress. 



Summary. 



1. Confirmation is given of the results of Loeb, Thacher, 

 Godlewski and Gast, and others, in showing that the hydranths 

 of hydroids (in this case Obelia and Campanularia) when 

 exposed to unfavourable conditions proceed to dedifferentiate 

 and to be resorbed, wholly or mainly, into the stem. 



2. Exposure to toxic agencies accelerates the process. Too 

 great concentration of poison kills the zooids before dedifferen- 

 tiation starts. Below the death-point, the acceleration is 

 proportional to the concentration. 



3. The effect is non-specific, both KCN and HgCla producing 

 the same result as prolonged exposure to laboratory conditions. 



4. When zooids are separated from the stem, resorption is 

 impossible. Dedifferentiation, however, proceeds until an ovoid 

 undifferentiated body packed with cells is produced. 



5. The tentacles are first affected, then the hypostome. In 

 early stages, separate tentacles may fuse locally. Stumps of 

 tentacles are, however, still present after the hypostome has 

 quite disappeared. The body becomes ovoid, then spherical, 

 and is finally reduced to a minute pigmented dot. 



