EMBRYONIC INDUCTORS AND ORGANIZERS 479 



induce neural development in ventral gastrula ectoderm isolated into the 

 solution.-^ '^ 



It was found by Fischer, Wehmeier, and associates that pure fatty 

 acids of animal and plant origin, also synthetic acids, emulsified in agar 

 and implanted, induce and that exhaustive extraction of amphibian em- 

 bryos or tissues of other animals with aqueous or organic solvents does not 

 abolish inductive power. Moreover, the nucleoprotein fractions derived 

 from these tissues are the active agents.^'' 



Induction, owing either to cephalin or some impurity, was obtained 

 with cephahn fractions of mammalian brain (Barth, 1934c). However, 

 acetone extracts of brain, containing sterols but no cephalin, and the un- 

 saponifiable fraction of cephalin also induce. Cephalin preparations that 

 induce also produce more or less cytolysis; and other cytolytic agents — • 

 for example, digitonin — with acid or alkahne buffers, also induce. The 

 protein residue of calf brain is more potent than the lipoid extract. ■'*' 



Since various tissues induce after killing but not while living, and cer- 

 tain foreign inductors — for example, cephalin and digitonin — produce 

 more or less cytolysis in the host, it has been suggested by Barth and 

 others that this cytolysis of host cells may set free inducing substance. 

 Neural induction by fuller's earth, silica, and CaC03 has been reported by 

 Okada (1938). These substances do not give off chemical inductors, but 

 they do produce injury and cytolysis of tissues. Okada regards substance 

 or substances set free by the cytolyzed tissue as the inductor in this case. 

 Induction by microcautery in the blastocoel is reported by Cohen (1938). 



Evidently the chemical problem of induction is by no means solved. If 

 induction or "evocation" is primarily activation, it seems not at all im- 

 probable that many chemical substances may be inductors and that irrita- 

 tion may also induce. It is also by no means certain that any of the for- 

 eign inducing factors is identical with the natural inductor. It has not 

 even been demonstrated that the natural inductor is a substance. In any 

 case, it is not evident how a particular chemical substance can organize 

 a neural axis unless its concentration or the amount set free is graded in 

 the inductor, but such a gradation provides an axial differential independ- 

 ent of the chemical constitution of an inducing substance. In any case, 

 induction by a particular chemical substance, whether produced by the 

 natural inductor or of other origin, throws no light on the problem of or- 



2* Beatty, De Jong, and Zielinski, 1939. 



^' Fischer, Wehmeier, Lehmann, Jiihling, und Hultzsch, 1935. 



i° Barth, igs4d, 1937, 1939a; Barth and Graff, 1938. 



