THE MOSAIC STAGE OF DIFFERENTIATION 237 



their tissues. This has been verified by experiments in which 

 epidermis from Amblystoma tigrinum is grafted into the appropriate 

 position on embryos of Amblystoma punctatum, and is found to be 

 incapable of forming a balancer. ^ 



Evidence regarding the existence of a nose-field is provided by 

 experiments on Rana temporaria in which the nose-rudiment is 

 extirpated at a stage prior to the formation of a nasal pit ; a nose 

 is nevertheless formed from neighbouring tissue, which grows over 

 to cover the wound. More distant epidermis will, however, not do 

 this. 2 The nose-field is therefore more extensive than the pre- 

 sumptive nose-rudiment, and if a large area, representing the entire 

 nose-field, is extirpated, no nose is formed. In some cases, the nose- 

 field gives rise to a single median nasal organ in place of the normal 

 paired two : this monorhiny is associated with and due to the same 

 causes as cyclopia (see Chap, ix, p. 348). 



§8 



Another case in which the presumptive zones or fields of different 

 organs appear to overlap is seen in the capacity which the adult 

 newt possesses of regenerating a lens to its eye. 



The material for the regenerated lens is derived from the dorsal 

 margin of the iris of the eye-cup itself.^ It will be remembered 

 (see p. 187) that in Rana esculenta the eye-cup retains for a con- 

 siderable time the power of inducing a lens, and that in many forms 

 when an eye-cup is grafted into the body of another embryo in such 

 a way that it is deprived of contact with epidermis, the eye-cup may 

 form a lens from its own margin.^ This appears to be what happens 

 in the regeneration of the lens in the adult newt. The eye-cup is 

 then of course separated from the epidermis by the cornea, and the 

 epidermis itself is differentiated into the conjunctiva ; the edge of 

 the eye-cup is represented by the margin of the iris. This power and 

 method of regeneration implies that the lens-inducing faculty and 

 the lens-producing faculty have not been lost by the eye-cup even 

 in the adult. 



The fact that it is always the dorsal margin of the iris which 

 provides the material for the regenerated lens requires considera- 



1 Mangold, 1931c. - Ekman, 1923. 



^ Colucci, 1891; Wolff, 1895. * Spemann, 1905 ; Adelmann, 1928. 



