124 CONTROL MECHANISMS IN CELLULAR PROCESSES 



ment, the competence to make hormones. The new competence 

 usually rests with the cells permanently, or at least is not easily 

 reversed, and therefore must be of a different nature than the tem- 

 porary gene activation due to inducers or hormones. 



Embryologists, since the ground breaking work of Spemann (1938), 

 are in continuous pursuit of the embryonic inductor, the one which 

 gives special competence to the cells. Many theories have been pro- 

 posed and rejected and even today, we are far from understanding 

 embrvonic induction. The problem has been discussed in several 

 recent reviews (Niu, 1956; Yamada, 1958; Tiedeman, 1959). 



All classes of substances have been proposed as embryonic in- 

 ductors, from glycogen and lipids to proteins and RNA. In recent 

 times, new hope that the real inductor would be finally discovered 

 arose with experiments of Niu ( 1956, 1958 ) . If the presumptive 

 ectoderm of an amphibian gastrula is cultured by itself, it does not 

 differentiate. If it is cultured with an organizer, usually the meso- 

 derm underlying the neural fold, it differentiates into neural tissue. 

 Niu observed that gastrula ectoderm, explanted into the culture 

 liquid in which previously an inductor was cultured, itself turns into 

 neural tissue. Some substance with organizing properties must have 

 been liberated by the previous culture. Various tests, especially the 

 use of specific enzymes, indicated that the substance was RNA in 

 Tritiinis torosus, but in T. rivulmis, it may have been a protein. 

 Works of other investigators ( Yamada and Takata, 1955; Englander 

 and Johnen, 1957; Tiedeman, 1959) showed that the inductor was 

 resistant to ribonuclease and that it behaved like a protein. Thus, 

 it is not possible yet to ascertain the role of RNA in embryonic in- 

 duction. Further discussion of the role of RNA in embryogenesis 

 can be found in a recent book of Brachet ( 1960 ) . 



Embryonic induction may be a process too complicated for easy 

 analysis. It would be advantageous to study a system where in- 

 duction changes one cell type into another, differing in only a few 

 characteristics and not in the ability to grow into an entire organ 

 system. In the development of the eye, the eye vesicle induces 

 lens formation in the overlaying ectoderm. Early in development, 

 this ectoderm is still nearly omnipotent and the induction by the 

 eye vesicle must activate sites specifically responsible for the pro- 

 duction of lens proteins, while repressing other potencies. In the 

 lens regeneration of newts ( Triturtts), retinal tissue induces new lens 

 formation from already differentiated pigmented epithelium of the 



