CHAPTER XII 



INDUCTORS AND SO-CALLED ''ORGANIZERS" IN 

 EMBRYONIC DEVELOPMENT 



THE demonstration by Spemann and his co-workers that t'he re- 

 gion of the urodele amphibian embryo which becomes the dorsal 

 lip of the blastopore and on invagination forms chorda-mesoderm 

 is a dominant region and can induce or determine development of other 

 parts provided experimental evidence in support of the conclusion that a 

 relation of dominance and subordination may be of fundamental signifi- 

 cance in the development of the amphibian embryo. Such a relation had 

 already been shown to exist for certain organs of later developmental 

 stages — for example, in induction of a lens by an optic vesicle or cup. 

 Dominance of certain parts and physiological or physical isolation from 

 that dominance had also been shown to be fundamental factors in agamic 

 and reconstitutional development (see chaps, ix-xi). However, the dis- 

 covery of a regional dominance and of the presence of a so-called ''organ- 

 izer" or "organization center" in amphibian development has exercised 

 a sort of dominance over the field of experimental embryology and has 

 resulted, during the last fifteen years, in a tremendous amount of investi- 

 gation on various aspects of the problem and in the discovery or postula- 

 tion of many other "organizers," concerned with one feature or another of 

 development, not only in amphibia but in many other organisms. In 

 chapters ix-xi it was shown that dominant regions in many reconstitu- 

 tions and agamic reproductions are apparently primarily the high regions 

 of gradients and that their dominance results from their activity rather 

 than from specific differentiation. Dominant regions resulting from sec- 

 tion and isolation alter existing gradients or determine new ones. In view 

 of the evidence concerning the relation of gradients and dominance to 

 axiate pattern, it appears probable that the gradient determined by an 

 activated region is the real organizing factor. According to this concep- 

 tion, the region of primary activation is an organizer only indirectly, by 

 initiating and determining a gradient pattern; conditions at different 

 levels of this pattern determine the orderly localization of parts along an 

 axis. In short, these experimentally determined dominant regions in the 



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