122 C. M. CHILD. 



acidity preceeds the disintegration gradient. Similar increase 

 in acidity preceding death has been observed in the cells of many 

 algae (Child, '16 a, b, '17). In many species of algee this gradient 

 in acidity is distinct within the length of a single elongated cell, 

 even though the staining may have been quite uniform up to 

 this stage. The axial relations of this increase in acidity are of 

 course the same as those of the disintegrative changes and the 

 color change of the dye may itself be used to a considerable 

 extent as a criterion of susceptibility. 



ACCLIMATION. 



Presentation of certain data which bear more or less directly 

 upon the problem of acclimation, e.g., the resorption and re- 

 development of hydranths, the transformation of apical regions 

 from hydranth production to stolon production and the reverse, 

 must be deferred to another paper. For the present it may be 

 noted that in low concentrations of various agents, KNC m/ 

 50,000-777/20,000, HC1 m/2, 000-772/1,500, L1C1 777/50, ethyl 

 urethane 777/2,000-777/500, MgSCX 777/500, neutral red and methy- 

 lene blue, indications of differential acclimation appear. In 

 the single hydranth under such conditions the body may die 

 before the tentacles, but in most hydroids the hydranths undergo 

 either disintegration or resorption so rapidly under slightly 

 depressing conditions in the laboratory that the investigation of 

 differential acclimation in the single hydranth is difficult. 



Attention has been chiefly directed, however, to the regional 

 differences in acclimation in the colony as a whole, which are 

 very definite and characteristic. In general, the physiologically 

 older hydranth shows less capacity than the younger hydranth 

 or the bud for acclimation to slight inhibitions, consequently, 

 in the course of a few days under such conditions, the older 

 hydranths disintegrate or undergo resorption, while the younger 

 persist or are affected to a less degree or less rapidly. Since the 

 order of arrangement of hydranths of different physiological age 

 along each axis and in the colony as a whole is definite and charac- 

 teristic for the species (pp. 112-117), the regional differences in 

 acclimation to low concentrations are in general the same as the 

 differences in susceptibility to lethal concentrations, that is, 



