110 DIFFERENTIATION III 



destroyed, the lens is not developed, and that the lens may 

 be developed from ectoderm taken from another part of the 

 body or even from another tadpole, and grafted over the 

 optic vesicle in place of the normal lens-forming cells, or from 

 the ectoderm lying over an optic vesicle transplanted to some 

 other region of the body. 



Again, in tadpoles grown in certain solutions (sodium 

 chloride, bromide, and nitrate) the optic vesicle may be some 

 distance from the ectoderm, and the lens is then absent, and 

 Stockard has found that in solutions of magnesium chloride 

 a single median eye may be developed in embryos of the 

 fish Fundulus instead of the two normal lateral eyes. Such 

 median eyes have a lens which is obviously not developed 

 from the ordinary lens- forming ectoderm. 



It seems therefore quite clear that under certain conditions 

 a lens may be formed over the optic vesicle but from other 

 cells than those usually devoted to its formation. On the 

 other hand it is urged that in certain cases lenses are un- 

 doubtedly developed from the usual cells and in the usual 

 positions, in spite of the absence of the optic vesicle (King, 

 Mencl, Stockard). 



Spemann has found that the lens, if formed under these 

 conditions, is small, but that by inserting an optic vesicle to 

 replace that destroyed, a lens of full size is produced. An 

 optic vesicle of Bombinator may even be used to replace one 

 of Rana. 



A fourth case is the formation of the cornea in Amphibia. 

 Spemann and Lewis have both found that in the absence of 

 the optic cup the corneal ectoderm does not * clear ' and that 

 Descemet's membrane is not formed. 



The development of the ear provides another instance. 

 The auditory vesicle or membranous labyrinth is formed first 

 and subsequently surrounded by the cartilage of the periotic 

 or auditory capsule. The latter, of course, conforms to the 

 shape of the former. The vesicle may be removed in an 

 early stage and transplanted to another individual. Here 

 it continues its development, and is surrounded by a capsule 

 developed from connective tissue which would of course 



