THE AXIAL GRADIENTS IN HYDROZOA. 2 1 



duction of KMnO 4 but when the reaction proceeds to completion in 

 excess of KMnO 4 a gradient in depth of color indicating differ- 

 ences in amount of reduction is evident. For demonstration of 

 these differences the material is kept in excess of KMnO 4 solution 

 24 to 4* hur> and is then dehydrated and cleared since it is opaque 

 in a<|u<-<>u> medium. Such cleared preparations are not permanent 

 in any .if the clearing agents thus far used nor in balsam. The 

 l.n.wn <.r blarkMi color due to MnO, or other oxides gradually 

 disapp' .r -ntly by solution, in the course of several days 



l.n-.-r. '1 he preparations are sufficiently permanent, however, 

 to permit imbedding and sectioning before the color gradient 

 disappears, though of course fading occurs more rapidly in the 

 section- than in the whole preparations. 



For demonstration of the gradients by methylene blue various 

 concentrations, ranging from i/i.ooo to I/ 20,000 were used, ac- 

 con <-s and result to be attained, the higher concentra- 



tions being UM-<! chiefly for differential staining, rapid death and 

 disintegration, the lower for demonstration of the gradients by 

 differential : n and slow disintegration. Concentrations 



of neutral red used were all very low but were not determined 

 because tl Its differ only as regards time with all concentra- 



tions ustd and because the dye gradually crystallizes out of 

 solution. 



Tin 1'ui.Ak GRADIENTS DURING NORMAL DEVELOPMENT. 

 The course of early development under normal conditions is 

 very similar in the species used as material. Cleavage gives rise 

 to a spherii-al ciliated blastula (Fig. i) which soon begins to 

 ws polarity in the graded thinning of the cell 

 wall decreasing from apical to basal pole, and in locomotion with 

 the apical end in advance (Fig. 2). Immigration of cells from 

 the basal hemi-phrre, chiefly from regions near the basal pole, 

 begins within a few hours (Fig. 3) and the cells which immigrate 

 apparently un-lergo further division and arrange themselves as an 

 inner entmh nnal layer as elongation of the larva continues (Fig. 

 4). The final form of the swimming planula, attained in Phiali- 

 ({iiaii after about forty-eight hours at laboratory temperature, is 

 approximately that shown in Fig. 5, but a considerable amount 



