Plate IV. Rat-kidney tubule sections (fixative 80 percent alcohol), dem- 

 onstrating the Gomori-Takamatsu technique for alkaline phosphatase. 

 Figure A, incubated 20 minutes. Compare this figure with Plate III, Fig. B. 

 By comparison the nuclei of the alcohol-fixed section are grossly precipi- 

 tated, and during fixation phosphatase has diffused from the cell borders 

 into the cytoplasm. Figure B, incubated 5 minutes: the main sites of 

 phosphatase are already apparent. Figure C is a section containing phos- 

 phatase superimposed on a section in which phosphatase has been destroyed. 

 Incubation time, 320 minutes. The active section has been grossly over- 

 incubated and appears almost opaque. The microscope was focussed on 

 the underlying section, which is devoid of phosphatase activity, but which 

 shows a faint image by diffraction. The absence of any apparent phos- 

 phatase activity in the underlying section shows that, during the incubation 

 period of 320 minutes, no significant amount of phosphatase or of calcium 

 phosphate has diffused into the underlying section, so that there is clearly 

 no significant diffusion artefact. Figure D is the same as Fig. C, but incu- 

 bated 640 minutes. By this time some of the nuclei of the underlying 

 inert section appear black, showing that either calcium phosphate or phos- 

 phatase has diffused into the underlying section. The intensity of staining 

 of the nuclei of the inert section has been deliberately exaggerated in the 

 photograph to provide contrast with the unstained background seen by 



diffraction. 



