132 INDUCTION AND ORGANISATION 



vesicle wall can still replace each other. The final determination 

 of the lateral wall to form a retina does not take place until 

 the beginning of the invagination of the vesicle. These experi- 

 ments make it plausible that induction from the invaginating 

 lens-vesicle co-operates in this determination. 



Let us now turn to the development of the lens. Transplant- 

 ation experiments have proved that in some amphibians the 

 ectoderm that will later form the lens has a weak lens-forming 

 potency already at the late neurula stage. If it is grafted, e.g. 

 into the ventral side, it will produce more or less well developed 

 small lenses (Harrison, 1920; Woerdeman, 1934). This potency 

 becomes much stronger at the time when the protruding eye- 

 vesicle comes into contact with the ectoderm. The assumption 

 was obvious that an inductive action from the eye-vesicle on 

 the overlying ectoderm plays a role here. The following experi- 

 ments proved that this hypothesis was correct. (1) In many 

 amphibians, no lens is formed if the eye-vesicle is removed 

 before it establishes contact with the ectoderm ; in other species, 

 only a slight thickening of the epidermis will occur, and in 

 only a few (e.g., Rana esculenta) a well developed lens is 

 formed, apparently independently of the eye-vesicle (Spemann, 

 1912), (Fig. 49). (2) If the lens ectoderm is replaced by other 

 ectoderm, originating, e.g., from head or belly, a lens will under 

 certain circumstances be formed by this foreign ectoderm. 

 (3) The same thing happens if an eye- vesicle is cut out, and 

 grafted under the ectoderm of another part of the body (Lewis, 

 1904), (PI. IX a, facing p. 136). 



However, in the last two experiments, it appeared that the 

 reaction of the ectoderm is not always the same in this respect. 

 At early stages, it is true, the whole ectoderm still has the 

 power to react to induction from the eye-vesicle with the 

 formation of a lens. But at least in some amphibians, after 

 neurulation this capacity becomes restricted to the more or 

 less immediate neighbourhood of the eye. The area of the "re- 

 action-field" decreases in the course of development. The 

 capacity to react, the competence, remains strongest at the 

 place where the lens is normally formed, and decreases 

 gradually from this centre toward the periphery. 



