THE AXIAL GRADIENTS IN HYDROZOA. 



33 



olism is higher than in early stages. The reduction gradients 

 with methylene blue appear only in relatively low concentrations 

 or after removal to water before staining has progressed too far, 

 but the concentrations and periods of staining suitable for this 

 purpose differ with the different species. For example, the plan- 

 uhe of Pliialidium show differential staining with the high regions 

 of the gradient or gradients most deeply stained in the early stages 

 of staining with methylene blue i/iooo, but in 1/5000 scarcely a 

 trace of differential staining occurs, the whole larva staining 

 more or less uniformly. In 1/20,000 there is no appreciable 

 staining during the first two hours of exposure, but after six 

 hours many planulae show a reversed staining gradient. In the 

 shorter planulae the depth of staining is least at the apical end of 

 the larva and increases basipetally ; in the longer planulae, in which 

 st.-,nd gradient is already present, both ends are slightly or 

 n< t at all stained and the depth of color increases toward the 

 middle region. Similar reversal of the staining gradient has 

 U-en ol 1 with low concentrations of methylene blue in vari- 



ous multicellular forms and in ciliate infusoria, including Parame- 

 (;<>. These reversed staining gradients in low concentrations 

 re-nit t":-"tn reduction of the dye to the leuco-compound in the 

 proti ipla-in. Below a certain concentration, which must be de- 

 termined experimentally for each species, the higher levels of a 

 gradient K duce the dye as rapidly, or almost as rapidly as it 

 enters and more rapidly than lower levels. Consequently the 

 depth of staining is least at the high end and increases toward 

 f a gradient. Even with concentrations high enough 

 I- stain the high regions of the gradients more deeply the reduc- 

 tion -: tg often appear when the animals are returned to 

 water before the staining has progressed far. Apparently the 

 reduced dye is toxic, for death and cytolysis finally occur in con- 

 centrations so low that the high regions of the gradients which 

 disintegrate first are only slightly or not at all stained. It is 

 improbable that the toxicity of such concentrations, e.g., 1/20,000, 

 can be due to impurities. 



CONCLUSION. 



The evidence from the study of the gradients during develop- 

 ment agrees with the data of observation in showing, first that 



