256 The Nature of Biological Diversity 



unsaturated alcohol, farnesol (Wigglesworth, 1961b). The same is 

 true for the 20-carbon unsaturated alcohol, phytol, according to un- 

 published results of experiments which I have performed in collab- 

 oration with John Law. However, in our experience, these well-known 

 chemicals are only about one-thousandth as active as the juvenile 

 hormone itself. 



Despite persistent uncertainties as to the chemistry of juvenile hor- 

 mone, we can state with considerable assurance that the function of 

 the hormone within the living insect is to block the flow of fresh 

 genetic information from nucleus to cytoplasm (Williams, 1961a, b). 

 By analogy with the simpler and more accessible microbial systems, 

 we find no dearth of mechanisms which could account for the con- 

 servative action of juvenile hormone. It could affect one or more 

 systems of positive or negative feedback concerned with the repression 

 of specific regulators, operators, or operons; it could interfere with 

 the flow of messenger RNA from nucleus to cytoplasm; it could 

 selectively block the utilization of this information in the synthetic 

 factories. 



This much we can say with confidence. Juvenile hormone somehow 

 prevents the cytoplasm from receiving or acting on fresh instructions 

 whose ultimate source must be the coded genetic information of the 

 nucleus. Meanwhile the hormone fails to interfere in any way with 

 the use and reuse of the information already at the disposal of the 

 cytoplasm. In the presence of juvenile hormone a cell can read and 

 reread the same chapter in the construction manual. But it cannot 

 press on to the next chapter. 



Insects and microbes 



So, in this brief excursion into entomology, we find that the insects 

 obey and in some instances illuminate a number of concepts of de- 

 velopmental biology derived from studies of microbial systems. These 

 concepts, without exception, have to do with the management of 

 genetic information — its de-repression and flow from its source in the 

 hereditary material to its implementation in specific synthetic acts — 

 more particularly, the synthesis of specific enzymes and structural 

 proteins. 



In insects as in microbes, it is increasingly clear that cells do not 

 continuously express all the potentialities inherent in their genetic 

 equipment. At any particular cross section of time, most cells are in 

 a highly repressed state. 



A bacterial cell synthesizes only those enzymes that are needed and 



