Introduction 



15 



Fig. 4. Besults of grafting belly epithelium from a toad em- 

 bryo (Bufo vulgaris) onto the eye region of an embryo of a frog 

 (Eaiia esoulenta). Here the grafted epithelium has formed, due to 

 its spatial relationship to the eye-cup of the host, a lens. Since 

 eyes are never normally present in the belly of the toad, this result 

 shows that belly epithelium possesses a potency (the lens-forming 

 potency) which, while never normally expressed, can be evoked by 

 suitable circumstances. The species characteristics of the transplant 

 (melanotic pigmentation) remain determined by the source, but the 

 morphological characteristics have been determined by the host. At 

 the right, cross section of entire head region. At the left, detail 

 of the right-hand eye showing the transplant. (From Filatow, D. 

 1925. Arch. entw. mech. Org. 105: 475-482, 402.) 



