DIFFERENTIATION 677 



organiser in a different relationship. Besides the main organiser 

 there are a number of subsidiary ones ; the optic cup, for example, 

 induces the formation of a lens in the tissue above it. Where an 

 organiser from one species is grafted into the embryo of another 

 it may still be able to induce organ development, but the details 

 of the tissues produced, as distinct from their type, are those 



t^" ':•- -;r 



Fig. 530. — Newt monsters, formed by grafting together two gastnila halves. The 

 anterior regions (seen in ventral view) have a plane of symmetry at right angles 

 to that of the posterior regions (seen in side view). The monster on the right 

 has a single eye. — From Huxley and De Beer, after Wessel. 



not of the organiser but of the host, showing that the genes of 

 the latter are still active. 



The mechanism of action of the organiser is obscure. There 

 can be no doubt that its cells have acquired some properties 

 which are not easily lost, for if all the cells of a gastrula of 

 Amhly stoma are separated by treatment with mild alkali, stirred, 

 and then neutralised, they will rearrange themselves with the 

 mesoderm going to the interior and the ecto- and endoderm- 

 forming surface epitheha ; organs such as the notochord, somites 



