Kidney stained with Polak?檚 silver method, light micrograph. Polak's silver method is used to demonstrate mitochondria in the kidney. Although the magnification of this image is too low to observe the mitochondria, the plane of section allows the medullary rays or Ferrein's pyramids that rise from the medulla and define the kidney lobules to be seen clearly. The boundary between the cortex, with kidney corpuscles in the form of dense rounded masses, and the outer medulla (occupying approximately the lower third of the image), which lacks these renal corpuscles, can also be clearly distinguished.