Chapter 1 13 anion visibility in the electron microscope being rendered by the electron opacity inherent with the 12 tungsten atoms present in each bound anion. Staining with PTA is quite fickle because all but the lightest rinsing of the grid-mounted sections with water causes the stain to desorb from the section. Much has derived from an examination of the effects of trypsin and pronase on the endocuticle.29 From a gravimetric analysis of the effects of these enzymes on physically isolated cuticle, the amino acid analysis of the solubilized components and examination of the enzyme treated sections in the transmission electron microscope, clearly defined sub-components of the endocuticle have been identified.29 A component rapidly removed with trypsin produced small holes in discrete regions of the endocuticle and with an amino acid composition strikingly rich in the basic amino acids histidine, lysine and arginine. It is believed this material was derived from the Figure 7. Transmission electron micrograph of a transverse section of human hair stained with phosphotungstic acid. Note here the very different character of staining in the cuticle as compared with the silver staining shown in Figure 4. Inset shows details of the cuticular cell membrane complex.