488 J. S. HUXLEY AND G. E. DE BEER 



is allowed to touch any hard object (Thacher, 1903). Clearly- 

 then, contact cannot be the only cause of resorption. It is 

 rather to be interpreted in terms of equilibrium between two 

 systems with different physiological reactions : if one wishes 

 to use Child's phraseology one may say that they possess 

 different metabolic rates, and the one with the higher rate is, 

 normally, physiologically dominant over the other. 



In the case of these hydroids there are two systems, the zooid 

 and the stem (hydrocaulus). 



Normally, the more highly differentiated zooid is able to 

 maintain itself, but being more specialized and less plastic 

 than the stem the zooid will not be able to maintain itself in 

 the face of conditions which, though adverse for the zooid, 

 do not appreciably affect the stem. Such are, e. g., a N/256,000 

 solution of KCN, or laboratory conditions after fifty hours. 

 The result of the adverse conditions is inhibition of normal 

 function ; and within limits it is differential, affecting the zooid 

 before and more than the stem. 



By interfering with general metabolism, as is done by exposure 

 to toxic agencies, the output of energy is reduced. Energy is 

 needed to maintain differentiated form against surface-tension. 

 Thus one of the first results of non-lethal interference will be 

 the loss of typical form by cells and their reversion to a 

 spheroidal or cuboidal shape. This is found in all cases of 

 dedifferentiation known, and often leads to the assumption of 

 spheroidal form by the whole organism (see Huxley, 19'2'2). 



The fact that exposure to laboratory conditions, to KCN and 

 to HgCla, all bring about identical reactions indicates that the 

 effects of the poisons, &c., are not specific, but that all act in 

 a general way, by affecting the energy-production of the tissues. 



It may be asked how the process we have called dedifferentia- 

 tion in Obelia differs from simple degeneration. The answer 

 is to be found in observation of the process. At no time can 

 it be said that the zooid is dead ; during the whole process of 

 resorption what is left of it is just as alive as the normal zooid 

 or stem. If the zooid dies, as it does if the poisons are too strong, 

 the cells acquire a characteristic semi-opaque appearance 



