Besides its action upon the sex cells and the brain cells, the internal secretion of the adrenal cortex acts upon the pigment cells of the skin, blunting their sensitiveness to light.

Pigment deposit in the skin as protection against light is controlled by the adrenal cortex.

The medulla of the hair contains the pigment granules or coloring matter, which may be of any shade between a light yellow and an intense black.

This pigment layer and that of the choroid and retina absorb the light entering the eye, so that little is reflected.

Owing to their rapid vibration, it is almost impossible to distinguish them while the spore is in active motion, but their effect is very plainly seen on adding colored pigment particles to the water.

Each Volvox-monad, moreover, possesses a red pigment spot, like the simplest form of eye known among animals.