IO2 C. M. CHILD. 



and following this the yellow color gradually disappears in most 

 cases, the dye being no longer held by the dead protoplasm. The 

 vital dyes, neutral red and methylene blue have been used both 

 as agents for staining intra vitam and for determining differences 

 in susceptibility. In the investigation of acclimation in lower 

 concentrations of KXC, HC1, ethyl urethane, MgSO,, neutral 

 red and methylene blue, it was found that different hydranths 

 or regions of the colony show differences in capacity for acclima- 

 tion. 



The question of the value of susceptibility as a criterion of 

 certain quantitative aspects of physiological condition has been 

 discussed so many times in earlier papers (see, for example, 

 Child, '196, and the references there given), that no further 

 consideration of the matter is necessary here. It need only be 

 said that a rapidly increasing volume of evidence along many 

 different lines indicates that in general susceptibility to higher 

 concentrations or intensities of at least many agents, as indicated 

 by death or inhibition, as well as capacity for acclimation to, 

 or recovery from the action of lower concentrations or intensities, 

 vary in general, though not necessarily proportionally to the 

 rate at which fundamental physico-chemical processes are going 

 on in protoplasm. 



Since adequate presentation of the experimental protocols on 

 susceptibility would require much space, only the general re- 

 sults are stated, but for each statement made there is an exten- 

 sive experimental basis. Moreover, the experiments demand no 

 difficult technique and can easily be repeated, the chief pre- 

 caution to be observed being the distinction between freshly 

 collected material in good condition and that which has been 

 kept in the laboratory. With some species even a tew hours 

 under laboratory conditions are sufficient to alter the suscepti- 

 bility relations of different parts to a considerable degree. 



THE DEATH GRADIENTS IN HVDRAMIIS. 



The observations on hydranths have been made repeatedly, 



both on the different, hydranths of single colonies and different 



olonies of the species mentioned above, and tin- progress of 



disintegration has been observed in hundreds of cases. The 



