486 PATTERNS AND PROBLEMS OF DEVELOPMENT 



cific actions, has been raised and discussed above. With the progress of 

 experiment evidence for specific action of the inductor seems progressively 

 less conclusive, even though specific substances may be concerned. The 

 data on neural induction do not conflict with the evidences of gradient pat- 

 tern given by other hnes of experiment. On the other hand, they may be 

 regarded as throwing some light on the manner in which development 

 proceeds from gradient patterns of certain sorts. 



An interesting question in relation to the problem of organization is 

 that of the origin of the inductor region. In amphibian embryos under 

 experimental conditions a region of invagination may appear in other than 

 the normal position, but evidently in relation to a pattern (pp. 259, 429). 

 These cases suggest that in normal development the dorsal inductor may 

 originate within a more general pattern, perhaps primarily as a local ac- 

 tivation, perhaps as the earliest determination or differentiation; but how 

 its localization on one side of the egg is determined is not known (p. 686). 

 That the inductor region is not a primary feature of pattern is also indi- 

 cated by the appearance of new inductors in reconstitution of isolated 

 extraembryonic parts of the fish blastoderm. The problem of organization 

 in vertebrates involves not merely the results of inductor action but the 

 origin and pattern of the inductor and the origin and nature of the pattern 

 within which it appears and acts. 



The question of the role of induction in normal development is not fully 

 answered as yet. In the teleosts isolation of a part of the blastoderm is suf- 

 ficient to bring about development of new embryonic axes in the entire 

 absence of the inductor. This being the case, does the inductor play any 

 essential part in normal neural development? Under certain conditions 

 explants of presumptive amphibian epidermis can develop into neural tis- 

 sue or even into mesoderm in absence of the inductor, though such devel- 

 opment probably results in some cases from activating conditions or sub- 

 stances which perhaps should be regarded as inductors. As regards birds 

 and mammals, we know at present little more than that new embryonic 

 axes may result from implantation of certain parts. 



If induction is essential to the development of a morphological axis in 

 the ectoderm and if it is primarily an activation, it is an activation in a 

 definite graded axiate pattern, and this is the first step toward axiate de- 

 termination and differentiation. Induction by the chorda-mesoderm does 

 not represent the origin of axiate pattern but is merely an expression of 

 pattern already present. That cells or regions at different relative levels 

 of physiological activity behave difTerently in development appears cer- 



