Regeneration 1^3 



organism does in this case what it is compelled to do 

 by its physical and chemical structure. 



Uhlenhuth' 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 trans- 

 parent 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 epithelitim ; and that 

 the ingrowing part will always become transparent. 

 This leaves us then with one puzzle still: Why is the 



' Not yet published. 



