Genie Control of Development 327 



had been produced and the inductor substance had been hberated. 

 Holtfreter calls it the X-substance. By grinding up and centrifuging 

 the cells, Holtfreter showed that the substance must be contained in 

 the cytoplasmic fraction of the centrifugate (not the lipids, etc.). 

 From such a fraction Brachet isolated extremely small granules which 

 also acted as inductors. They contained RNA and their action was 

 inhibited by ribonuclease, from which it was concluded — in harmony 

 with the other ideas of Brachet reported above — that RNA is the 

 inductor substance. Without accepting this as certain, Holtfreter as- 

 sumes that the active substance X is present in all cells but bound 

 to another moiety. In all types of inductor experiments, the substance 

 X is liberated and can act. 



The latest contribution to the subject by Niu and Twitty ( 1953 ) 

 agrees generally with all these facts. They showed that an ectodermal 

 explant (in tissue culture) undergoes induction when mesodermal 

 tissue has been present for some time in the drop, without any con- 

 tact between the two. Thus induction at a distance, by a diffusible 

 substance, occurs; preliminary tests indicate that the substance is a 

 nucleic acid. 



These are the decisive facts. Our problem in the present dis- 

 cussion is to put the inductor action in the proper place within the 

 embryonic determination system of genically controlled processes of 

 cytoplasmic differentiation in time and space by a nucleo-cytoplasmic 

 interaction which we called genie activation. We must start with the 

 fact that orderly development in large groups of animals is carried 

 out without the assistance of an inductor system. Thus the latter is an 

 addition to the basic system of nucleo-cytoplasmic relations controlling 

 orderly development which we studied thus far. It is obvious that the 

 inductor method does not replace the ordinary method, which accom- 

 plishes differentiation by some action of the genie material upon cyto- 

 plasm competent for the interaction. This is proved by the following 

 facts : ( 1 ) development of the inductive type takes place with continu- 

 ous narrowing down of the competences for inductor action, which 

 are first available everywhere and subsequently are restricted to 

 smaller and still smaller areas; (2) induction initiates the general type 

 of organ differentiation, but does not affect the genetic specificity 

 residing in the cells, which differentiate under the influence of the 

 inductor, nevertheless, according to their own genetic origin; (3) 

 the same inductor produces different differentiations according to the 

 region into which it is implanted. The induced organs are, with a cer- 

 tain variation, those belonging to the respective region (e.g., eyes in 



