474 J. S. HUXLEY AND G. R. DE BEER 



The Hydrozoa are not virgin soil to the experimentalist in 

 this connexion. Loeb (1900) observed that under certain con- 

 ditions the zooids of a hydroid colony would lose all their shape 

 and structure and retreat into the hydrocaulus. He attributed 

 the cause of this to contact with solid objects, viz. the watch- 

 glass in which the organisms were kept ; but this explanation 

 is probably not correct. 



Thacher (1903) investigated the process of retrogression in 

 hydroids and called it ' absorption '. Cerfontaine (1902) says 

 of it : ' les individus . . . degenerent et disparaissent.' Gast 

 and Godlewski (1903) merely called it ' degeneration ' (' Eiick- 

 bildungsprozess '). These terms are inadequate to designate 

 a process as specific as that which Huxley (1921 h) has described 

 in the case of Perophora. Strictly speaking, there are two 

 processes at work, viz. dedifferentiation and resorption. In 

 the following description of the experiments performed Resorp- 

 tion will be used to mean the process whereby the material 

 composing the zooid is transported, and Dedifferentiation 

 to mean other processes undergone involving a return of cells 

 or tissues to a simpler, less differentiated condition. 



In the organisms chosen for experiment there is a coexistence 

 of two sets of systems, the ' zooid systems ' and the ' stolon 

 systems '. It is obvious that normally physiological equilibrium 

 must exist between them. But if circumstances can be found 

 w^hereby one system is adversely affected more than the other 

 there will occur differential inhibition associated with resorp- 

 tion or dedifferentiation or both. 



The experiments were performed at the Biological Laboratory, 

 Woods' Hole, in 1916 (Campanularia, J.S.H.), and at the 

 Marine Biological Laboratory, Plymouth, in 1920 (Obelia, 

 G.R. de B.). 



Experimental. 



Given the fact that mere subjection to unfavourable con- 

 ditions, viz. being kept in glass vessels in the laboratory, brings 

 about resorption of zooids in hydroids, it was to be expected 

 that if the toxicity of the water were increased, the process of 



