organisers: inducers of differentiation 183 



development, in tadpoles arising from eggs kept in urea solutions, 

 of patches of tissue, within the nerve-cord or the gut, whose histo- 

 logical structure is identical with that of the notochord. These 

 patches of ectopic notochordal tissue are always adjacent to the 

 true notochord. It would appear that there has been some spread 

 of the factors responsible for this particular histo-differentiation, 

 possibly by the diffusion of specific substances from the notochord- 

 rudiment. As Lehmann {Naturwiss. xxi, 737) has recently shown, 

 lithium treatment results in differential reduction of the trunk- 

 notochord in Triton. 



Fig. 87 

 Spread of notochordal type of histo-differentiation to neighbouring organs in 

 frog tadpoles reared in 1-5 per cent. urea. Left, notochordal differentiation in the 

 gut-roof. Right, notochordal differentiation in the nerve cord. Below the noto- 

 chord in each case is the sub-notochordal rod. (Redrawn after Jenkinson, Arch. 

 Entiumech. xxi, 1906.) 



§8 



We have left to the last what is the most celebrated example of de- 

 pendent differentiation — the formation of the lens of the vertebrate 

 eye from the epidermis under the influence of the eye-cup (fig. 86). 

 The matter, however, is not simple, and is worth going into at 

 some length. 



In Rana temporaria {fusca) the lens is dependent for its develop- 

 ment on contact with the eye-cup. If the latter is removed (at 

 the tail-bud stage), the lens is not formed. ^ Further, the eye-cup in 

 this species is capable of inducing the formation of a lens out of 



^ Spemann, 1901A, 1905. 



