SUSCEPTIBILITY IN HYDROIDS. 121 



ferent parts of'the hydranth are slight or only temporarily visible 

 in the earlier stages of staining, but the more extreme differences, 

 such as those between hydranths and stems, remain visible for 

 a longer time, in fact the hydranths may be killed by the stain 

 before the stem is very deeply stained. In general, however, 

 the differences which appear at first tend to become equalized 

 as staining progresses. Apparently the rate of accumulation 

 of the dye decreases most rapidly in those cells which at first 

 stain most rapidly, so that the early differences in rate of stain- 

 ing are obliterated, at least to a large extent, by the action of the 

 dye on the cells. 



Nevertheless the differences in susceptibility, as indicated by 

 survival time are very marked in these dyes, even in regions 

 where the staining appears to be uniform in degree. The early 

 differences in rate of staining are evidently not alone responsible 

 for the differences in susceptibility. In fact it is much more 

 probable, as has been suggested elsewhere (Child, 'i6b, Child 

 and Hyman, '19), that the conditions indicated by the differences 

 in staining rate, whether differences in permeability, adsorption 

 or some other factor, are simply one aspect of the differences in 

 fundamental physiological condition which determine differences 

 in susceptibility. The facts of acclimation briefly referred to 

 below, so far as they concern the vital dyes, show even more 

 clearly that the differences in physiological effect are not due to 

 the initial differences in rate of staining, for parts which stain 

 at first more rapidly show a greater capacity for acclimation than 

 less rapidly staining parts. 



As death approaches in cells stained with neutral red, a dis- 

 tinct change in the color of the dye toward the acid side occurs, 

 i.e., the cell contents become distinctly acid shortly before 

 disintegration. This change in color of neutral red occurs quite 

 independently of the nature of the killing agent, which may ) 

 for example, be neutral red itself, an acid, KNC which is alkaline 

 in solution, etc. Of course as soon as disintegration occurs the 

 color of the neutral red is determined by the hydrogen ion 

 concentration of the entering solution. The increase in acidity 

 preceding death is sufficient to be clearly visible and in elongated 

 axiate parts, e.g., the tentacle or hydranth body, a gradient in 



