220 PRINCIPLES OF EMBRYOLOGY 



There is still very much to learn about the origin and loss of competence, 

 and its quantitative intensity. It is, for instance, difficult to beheve that all 

 competences throughout development can arise autonomously, without 

 dependence on previous inductive processes. There w^ere indeed some 

 indications in Waddington's experiments that the lens competence is not 

 completely autonomous, since it seemed only to appear if the isolated 

 ectoderm remained as a fairly thin sheet, and to be absent from thick 

 solid masses of tissue (for discussion : see Waddington 1940^). Another very 

 interesting problem is that imphcit in the use of the phrase 'weaker 

 reactions' above. Is the decision between the sub-alternatives dependent 

 on the strength (i.e. intensity and/or duration) of stimulus? and can varia- 

 tions of this kind perhaps even tip the scales between the major alterna- 

 tives of neural tissue and mesoderm ? We shall return to this question in 

 discussing individuation of the embryonic axis (Chapter XX). 



It must not be overlooked that competence involves the capacity for 

 self-individuation leading to the formation of a well-shaped organ. In the 

 normal development of the neural system, the inducing archenteron 

 roof undoubtedly plays an important part in the individuation of the 

 neural system, but quite well-organised differentiation can occur even 

 when the inducer is certamly structureless and unable to contribute to the 

 result. 



Although so little progress has been made with the embryological 

 study of the waxing and waning of competence and the factors which 

 bring it into being, there is another line of approach to which we may 

 turn. Granted that competence is a state of instability in a complex system 

 of reactants, what may we suppose these reactants to be? Now genetics has 

 taught us that the characters which an egg develops are ultimately con- 

 trolled by the genes contained in its nucleus. The various processes which 

 may or may not proceed, according as the instability is resolved one way 

 or the other, must therefore be gene-controlled; and the reactants which 

 give rise to the competence must be the genes or at least factors dependent 

 on the genes. Evidence of this may be seen within the organiser pheno- 

 menon itself. We have mentioned several times that organiser grafts may 

 be active even if made between quite different species. If ectoderm from 

 a species of Urodele with a large egg is grafted so that it comes to he over 

 the mesoderm of a small egg of another species, it is found that only a small 

 neural plate is formed; there is an adjustment to the size of the inducing 

 organiser out of which the evocator diffuses (Holtfreter 1935). But' this 

 is almost the only respect in which such an adjustment occurs. As regards 

 nearly all other characters but size, the induced organs have the character- 

 istics of the species to which the competent tissue belongs (for discussion: 



