THE AXIAL GRADIENTS IN HYDROZOA. 79 



and the physiological gradients have been followed through all 

 developmental stages by various methods, but consideration of 

 the embryological data is postponed to a later paper. Among the 

 hydromedusae four species, PhiaUdium gregarium, Mitrocoma 

 discoides, ^Equorea ccerulescens and Sarsia rosaria constituted the 

 chief material for the work with permanganate, but observations 

 were made as opportunity offered on several other forms, Stoma- 

 toca, Obelia and several undetermined species. 



The experimental procedure consists merely in immersing the 

 species to be examined in a solution of KMnO 4 in sea water: 

 For observations on the color gradients resulting from differences 

 in rate of reduction concentrations ranging from w/iooo KMnO 4 

 down to m/i 0,000 or even less have been used in most cases. 

 Higher concentrations than w/iooo may of course be used, but 

 in such concentrations the color of the solution is so deep that 

 the animals must be removed to water for observation. More- 

 over, the differences in rate of reduction and staining are more 

 distinct in the lower than in the higher concentrations because 

 the latter kill all parts almost at once in spite of the differences 

 in physiological condition. Differences and gradients in total 

 amount of reduction are determined by permitting the reduction 

 to proceed to completion in excess of permanganate. Under 

 these conditions most organisms become opaque black and no 

 differences in depth of color can be seen, but in certain small 

 organisms, embryonic stages, or regions of body, in which the 

 thickness of protoplasm or cell mass is not too great, the color 

 gradient resulting from differences in amount of MnO 2 deposited 

 is directly visible. Many other forms which are opaque black 

 in water may be made more or less translucent by hardening and 

 gradual dehydration in alcohol and clearing in an oil, and if 

 desired mounting in balsam. The axial gradients in blastulse 

 and other early embryonic stages of various species, as well as 

 in the adults of various small forms, can be seen with great 

 distinctness in such preparations. Larger organisms or regions 

 which reduce relatively large amounts of permanganate may 

 remain opaque even after clearing. In fact, the method is 

 chiefly of value for small, more or less translucent organisms, 

 but for these it gives very definite, uniform and beautiful results. 



