THE ORIGIN OF NEW CELLS AND TISSUES 2O5 



the new lens is that it arises from a part of the body from which the 

 lens of the eye never develops in the embryo of this form or of any 

 other vertebrate. In the embryo the lens develops from the ecto- 

 derm at the side of the head and only secondarily unites with the 

 optic cup, that has come from an evagination of the anterior wall of 

 the fore brain. In the regeneration of the adult lens, however, the 

 ectoderm covering the eye takes no part in the formation of the new 

 lens, in fact, it is separated from the eye by the thick inner, meso- 

 dermal layer of the cornea. The lens develops, as has been stated, 

 from the already differentiated layers of the iris. It is a point of 

 further interest to notice that the cells that form the transparent lens 

 come from the iris cells that are in part at least filled with black pig- 

 ment. If this pigment remained in the cells the new lens, while it 

 might be structurally perfect, would be physiologically useless. The 

 pigment disappears, however, as the lens develops. In this case we 

 find a highly specialized organ, the lens, developing out of tissue 

 also specialized in another direction. It does not simplify the prob- 

 lem to point out that the lens and the iris are both parts of the eye, 

 since they have arisen from different parts of the body and have 

 only secondarily come into apposition with each other. Colucci was 

 contented to point out that both the embryonic lens and the regen- 

 erated one come from ectoderm and that the result can be brought 

 into harmony with the " germ layer " hypothesis. 



Wolff has called attention to the fact that the new lens arises 

 from the upper edge of the iris, and that this is obviously the most 

 advantageous position in which it could develop from the iris, since 

 by its own weight it falls into place as it develops. If the lens had 

 developed from any other point of the margin, its position would be 

 less advantageous, as it might not be brought into its proper position. 



Fischel, who has more recently studied the regeneration of the 

 lens in the larvae of Salamandra maculata, finds that after the 

 removal of the lens the iris is thrown into wrinkles or folds and may 

 stick at first to the cut-edge of the cornea. After the cornea has 

 healed, the iris returns to its normal position. He finds that the 

 first changes are more or less alike around the entire rim of the iris 

 and involve a partial absorption of the pigment, a separation of the 

 inner and outer layers at the edge, and a swelling of the margin. 

 These changes go only a little way in those parts that do not pro- 

 duce a lens, but at the upper edge of the iris they go farther and 

 lead to the formation of a lens in that region. He finds also that a 

 new lens develops in animals kept in the dark as well as in those 

 kept in the light, and in the same way. 



Fischel also tried the effect of removing a part of the upper edge 

 of the iris at the time when the lens was extirpated, in order to see 



