142 organisers: inducers of differentiation 



an embryo of a species different from its own. We can go further, 

 and say that its action seems singularly non-specific. Not only can 

 an organiser from Triton cristatus function in Triton taeniatus^ but 

 also organisers from Pleurodeles waltli, Amblystoma mexicanum, 

 and even the Anuran Bombinator pachypus^ can induce the forma- 

 tion of secondary embryos in Triton taeniatus.^ It is therefore 

 established that the inducing action of the organiser is not impeded 

 by a taxonomic difference of the order of value of a sub-class be- 

 tween its own tissue and that on which it works (fig. 65). 



These experiments of heteroplastic and xenoplastic organiser 

 grafts between different species demonstrate the fact that the action 

 of the organiser is specific as to the general type of organs and 

 structures produced by induction, but non-specific as to the details 

 of these structures ; these latter are governed by local and intrinsic 

 properties and determinations of the tissues themselves, over which 

 the organiser has no control. For instance, a piece of presumptive 

 neural fold tissue of Triton taeniatus grafted on to the side of the 

 head of an embryo of Triton cristatus will differentiate into gills in 

 its new position. But, gills though they are, they preserve their 

 taeniatus character in being larger than the normal cristatus gills 

 on the other side of the embryo.^ Conversely (fig. 15), cristatus 

 tissue on Triton taeniatus gives rise to gills which are smaller than 

 the normal taeniatus gills.^ The retention of specific characters in 

 spite of induced determination to develop into structures other 

 than those which a piece of tissue would normally have produced, 

 is shown even more strikingly in those experiments in which a piece 

 of Anuran presumptive epidermis (from the ventral side of the 

 trunk) is grafted over the future mouth- region of a Urodele embryo. 

 In its new and strange position, the Anuran tissue differentiates into 

 mouth-parts, and it also gives rise to a ventral sucker of Anuran 

 pattern* which is functional and secretes an adhesive substance. It 

 also appears that horny teeth can be formed as well. No Urodele 

 normally possesses a sucker or horny teeth (fig. 66). 



As a further illustration, we may take the results of experiments 

 in which a Urodele organiser (from Triton alpestris) is grafted into 

 an Anuran embryo {Bufo vulgaris). The induced secondary embryo 



^ Geinitz, 1925 b; Schott^, 1930. ^ Spemann, 1921. ^ Rotmann, 193 1. 

 * Spemann, 1932, 1933; Spemann and Schotte, 1932. 



