Uhlenhuth[167] has shown by way of tissue culture that the cells of the iris cannot grow and divide as long as they are full of pigment granules as they normally are. When the fine superficial membrane of the iris is torn the pigment granules fall out and the cells can now grow and multiply. If the lens is taken out of the eye of the salamander the fine membrane of the iris is torn and the pigment cells at the edge (especially the upper edge) lose their pigment granules which fall down on account of their specific gravity. As soon as this happens the cells will proliferate. A spherical mass of cells is formed which become transparent and which will cease to grow as soon as they reach a certain size. The unanswered question is: Why does the mass of cells become transparent so that it can serve as a lens? The answer is that young cells when put into the optic cup always become transparent no matter what their origin; it looks as if this were due to a chemical influence exercised by the optic cup or by the liquid it contains. Lewis has shown that when the optic cup is transplanted into any other place under the epithelium of a larva of a frog the epithelium will always grow into the cup where the latter comes in contact with the epithelium; and that the ingrowing part will always become transparent. This leaves us then with one puzzle still: Why is the growth of the lens limited? The limitation in the growth of organs is one of the most important problems in growth and organ formation, though unfortunately our knowledge of this topic is inadequate. [167] Not yet published.