Eyebcdl formed from 

 outgrowth of 

 embryonic 

 brain 



from skin cells 



Inside of eye ■ 



formed from 



mesoderm ^^' 



INTERACTION OF DEVELOPING STRUCTURES 



The eyeball and retina develop as an outgrowth of the brain in the young embryo, 

 whereas the lens develops as an ingrowth of the skin. If the eye-bud of an embryo is 

 removed at an early stage and implanted under the skin on any other part of the 

 body, the skin cells will develop a lens, where it cannot possibly be of use 



Even young tissues act in specific ways. It is therefore possible that there 

 are several or many organizer substances. In any case, the evidence shows 

 that the parts of the embryo probably act on one another during develop- 

 ment through chemical substances. 



In some very clever grafting experiments Hans Spemann (1869-1941), a 

 distinguished German biologist, used embryos of two different kinds of 

 salamander. Spemann removed bits of shjn from the abdomen of a sala- 

 mander embryo and grafted it on the brain of one of the other type. In this 

 position the skin cells developed brain, but they retained the character of 

 their own species. Let us suppose that these developments were determined 

 by the presence of organizer substances. We should then say that the or- 

 ganizer in the brain region changed skin cells into brain tissue, while the 

 organizer in the skin cells determined the appearance or perhaps the pig- 

 mentation of the new (brain) cells formed. 



More striking are experiments in which organs are made to develop in 

 strange locations. In many vertebrates the eye is formed as an outgrowth of 

 the brain, at a very early stage. The lens, however, is formed by an in- 

 growth of the skin, above the eyecup — but it takes something in the eye- 

 bud to make skin cells form a lens (see illustration above). Again, the 

 external eardrum of the frog, which is easily examined, is formed by the 

 regular skin cells above a ring of cartilage. If this cartilage is removed in 

 the embryonic stage and grafted under the skin on the back or side of the 

 frog, the local skin will become thin and form the peculiar eardrum tissue. 



Thousands of experiments have been carried out on embryos of many 

 species. The results agree with the notion that particular substances are 



363 



