EMBRYONIC INDUCTORS AND ORGANIZERS 501 



synergetic principle in development. s" Development of the part is doubly 

 assured, on the one hand, by its own determination and, on the other, by 

 the inductor. It must be pointed out, however, that there is no double 

 assurance unless the part concerned can actually develop independently 

 of, as well as in consequence of, the action of the inductor. As far as we 

 know, a complete, normal neural plate does not develop independently of 

 an inductor. The neural tissue or neural tube developing independently 

 of an inductor could not serve an individual as a central nervous system. 

 Its independent development is without value to the organism. The lens 

 of some amphibian species apparently constitutes a better example of 

 double assurance, but in other species double assurance is apparently 

 lacking. In the teleost embryo under inhibiting conditions several lenses 

 may develop independently of an eye, seemingly a multiple assurance 

 but without significance for the normal organism. 



But even if it be granted that both independent differentiation and de- 

 velopment by induction are concerned in certain parts, the principle of 

 double assurance throws no light on the problem of the organization which 

 initiates independent development or of the factors concerned in induc- 

 tion. It is a teleological, not a physiological, principle. We must, then, 

 ask the question: What does double assurance mean physiologically? 

 Does it mean anything more than that the pattern within which a greater 

 or less degree of independent development or determination takes place 

 and the inductor act on the part concerned in very much the same man- 

 ner, though perhaps in different degree or intensity? Assuming, for the 

 moment, that the primary pattern of organization is a gradient pattern, 

 the presumptive neural plate represents a certain region, a certain range 

 of levels, in that pattern; and whatever independent development is pos- 

 sible results from its position in, and relation to, that pattern. The inductor 

 apparently represents a higher range of gradient-levels and, by raising 

 the level of presumptive neural plate, makes possible further development. 

 But apparently almost any stimulating or irritating factor acts in some 

 degree like the natural inductor, though those factors are usually without 

 axiate pattern, and any axiate pattern developing must be of ectodermal 

 origin. These suggestions do not necessarily involve the assumption that 

 the invaginated chorda-mesoderm differs only quantitatively from the 

 presumptive neural plate. It may already be specifically different when 



so Spemann, 1931, pp. 508-10; 1936, p. 59; 1938, p. 92. For earlier biological applications 

 of this principle see Rhumbler, 1897; Braus, 1914. For the principle of kombinative Einheits- 

 leistuHg, a somewhat broader concept than double assurance, see Lehmann, 1928, and later 

 papers cited in Bibliography. 



