EMBRYONIC INDUCTORS AND ORGANIZERS 461 



level. Are the longitudinal differences in induction and reactivity prima- 

 rily anything more than these gradient differences? 



The question of longitudinal regional specificity of the inductor has 

 been much discussed. The chorda-mesoderm is regarded as a mosaic of 

 specific inductors by Holtfreter, and Lehmann believes he has obtained 

 evidence of regional specificity by the use of LiCl.'^ In fact, Lehmann 

 holds that action of chemical agents on embryonic developmental stages 

 is, in general, regionally specific. As regards his experiments, however, 

 it must be noted that the possibilities of differential recovery and of altera- 

 tion of nonspecific differential susceptibilities in the course of development 

 by alteration of activity are completely ignored. 



This hypothesis of longitudinal regional specificity in the inductor pre- 

 sents difficulties. Neither inductor nor presumptive neural plate gives any 

 evidence of specific regional differences in earlier stages, though this does 

 not prove their absence. Moreover, different levels of the central nervous 

 system all differentiate into cells and fibers; the specificity of the different 

 levels appears to be in the relations of these rather than a chemical speci- 

 ficity, but how the inductor can determine these relations does not appear. 

 Head-inductor and trunk-inductor induce trunk or head according to level 

 of implantation, and head is not present or absent but shows various de- 

 grees of gradation between complete development and absence. This does 

 not suggest specificity of induction. Head and trunk, of course, become 

 widely different in later stages, and various other inductions occur in 

 their development; but that the neural inductor is anything more than a 

 factor in determining relative physiological levels in the reacting ectoderm 

 remains to be proved. The regional differences in the inductor are re- 

 garded as quantitative by Dalcq and Pasteels.'^ 



The inductor region may be specifically different from other regions 

 of the egg cytoplasm at the beginning of development; but, as will appear, 

 a great number of tissues, living and dead, tissue extracts, and chemical 

 substances are also neural inductors; consequently, any close relation be- 

 tween whatever specificity may be present in the natural inductor and 

 its inducing capacity does not appear probable. According to Holtfreter 

 (1939a), there is complete lack of specificity in aggregations of isolated 

 amphibian cleavage cells, but with isolation at later stages pure ectoderm 

 and pure entoderm may aggregate. 



'■* Holtfreter, 1938, and various earlier papers; Lehmann, 1936a, b, 1937a, c, 19386, c. 



'5 Dalcq, 1935, 1938a; Dalcq et Pasteels, 1938; Pasteels, 1938. See also Pasteels, 1938, 

 "Recherches sur les facteurs initiaux de la morphogenese chez les Amphibiens Anoures. I," 

 Arch. Biol., 49. 



