82 C. M. CHILD. 



Thus far an exact quantitative record of the differences in rate 

 of staining has not been attempted because of the difficulties 

 involved. It is difficult to determine exactly the moment when 

 the staining begins in a certain region and although the regional 

 differences in depth of color are very marked, even in forms as 

 small as the hydroid planula or sea urchin blastula and gastrula, 

 any adequate measure of this difference is not readily obtained. 

 Photomicrographs can undoubtedly be made of stages of staining 

 or of the cleared preparations, but in any case are little better 

 than figures, and graphic methods are much less readily applied 

 than in case of disintegration experiments (Child, '15, Chaps. 

 III. -VII.). In the present paper description is supplemented 

 by a few diagrammatic figures in which the regional differences 

 are indicated at some stage of coloration by degrees of shading. 



REDUCTION GRADIENTS IN HYDROID COLONIES. 



Before describing the details it may be said that in general 

 both the rate and, so far as it could be determined, the total 

 amount of reduction, decrease from the axial regions basipetally. 

 This holds not only for the single hydranths, hydranth buds, 

 growing tips, medusa buds and stolons, but for the colony as a 

 whole. The differences at different levels are least in the stems. 

 The perisarc is of no very great significance as an obstacle to the 

 passage of permanganate to the tissues within it. In the more 

 apical regions parts inclosed in perisarc begin to stain almost 

 or quite as quickly as naked parts, and even the thicker perisarc 

 of the more basal regions of the stem retards the staining only 

 slightly. The thickness of the perisarc, therefore, is certainly 

 not responsible for the differences in rate of staining of different 

 levels. The deposition of the oxide begins on the external 

 surface of the protoplasm and a distinct regional gradient in 

 staining is in many cases visible on the surface before any trace 

 of reduction appears below the surface. 



The Hydranth Gradients. -Reduction and coloration occur first 

 at the tips of the tentacles and within a few moments each ten- 

 tacle shows a distinct color gradient ranging from brown at the 

 tip to light yellow in the basal region. Color begins to appear 

 at the tip of the manubrium shortly after it appears on the 



