RNA AND CONTROL OF CELLULAR PROCESSES 123 



This evidence certainly supports tlie assumption that a complete 

 set of genes is present in everv cell. One of the primary points of 

 the present discussion is that it would lie uneconomical for all genes 

 to be active in the cell at all times. Therefore, in development, 

 gene activity must also be reo;ulatcd, and this regulation must be of 

 an orderlv character, not a chance activation and inactivation en- 

 tirely at the mercv of external influences. Each gene should become 

 activated at an appropriate time in development. This can be 

 achieved onh- if a signal indicating the developmental stage of the 

 organism is sent back to the cells, and also a signal indicating a 

 cell's position in the whole. These signals then would induce or 

 repress appropriate gene functions. What is their nature? 



Could differentiation depend on a system akin to enzyme induc- 

 tion? In some cases, this may be possible. As soon as a certain 

 substance appears in the medium, the corresponding enzyme could 

 be induced. However, it would be difficult to explain thus all the 

 facts and intricacies of development. What about inducing the for- 

 mation of proteins which are not enzymes, such as structural pro- 

 teins, collagen, keratin, or silk fibroin? Enzyme induction can be 

 found in higher organisms, but not as a regulatory mechanism in 

 dift'erentiation. Knox and his collaborators (1961) have found sev- 

 eral inducible enzymes in rabbit liver. One of these, trvptopliane 

 peroxidase, is induced by tryptophane and the process of induction 

 seems to be similar to that in bacteria. The system is more compli- 

 cated, really, since cortisone highly increases inducibility and may be 

 required for induction to occur at all. Besides, the cells can be in- 

 duced only after a certain developmental stage is reached, not before 

 the second dav after the birth of the animal. At that time something' 

 must have happened to the cell to give it competence for induction. 

 And finally, the inducer acts only on liver cells, not on other cells 

 of the organism. Inductive enzyme formation then is a special case, 

 not inherent in all cells with the same genetic constitution. It is the 

 induction of competence which is important, and this is an embry- 

 onic process, which must act on difi^erent principles than plain in- 

 duction. - 



Could embryonic induction be similar to the action of hormones? 

 Again, it is obvious that the production of hormones is restricted to 

 certain tissues and activation b\- other hormones acts on this tissue 

 only. The cells of these tissues must have acquired, during develop- 



