INSECT METAMORPHOSIS: AN APPROACH TO THE STUDY OF GROWTH 319 



hormone is present, the cells destined to die are prevented from acting 

 out their latent lethal programs. Thus the picture presented during 

 successive instars of larval life is one of generalized cellular growth 

 and vitality. 



The conservative action of juvenile hormone is documented in a 

 most dramatic way in experiments in which pupae are injected with 

 juvenile hormone. As previously mentioned, the pupa molts and pro- 

 duces not a moth but a second pupal stage. Dissection of these animals 

 reveals the integrity of the fat-body, the prothoracic glands, and all 

 other tissues that routinely break down in the course of adult develop- 

 ment. The presence of juvenile hormone prevents, so to say, the genetic 

 "count-down" on the cells that are programed to die. 



Mode of action of juvenile hormone 



According to this analysis, juvenile hormone is a conservative 

 agent which blocks the flow of fresh genetic information from nucleus 

 to cytoplasm. 



So little is known about the channelization and management of 

 information in animal cells that the analysis can scarcely be pressed 

 beyond this point at the present time. However, by analogy with the 

 siitipler and more accessible microbial systems, we find no dearth of 

 mechanisms which could account for the conservative action of ju- 

 venile hormone. The hormone could affect one or more systems of 

 positive or negative feedback concerned with the repression or de- 

 repression of specific gene combinations; it could interfere with nu- 

 cleolar action or the synthesis and coding of new RNA; it could oppose 

 the flow of new RNA from nucleus to cytoplasm; it could selectively 

 cover up and inhibit newly formed ribosomes. 



This much we can say with confidence: Juvenile hormone some- 

 how prevents the cytoplasm from receiving or acting on fresh instruc- 

 tions 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. 



References 



Bounhiol, J. J., 1938. "Recherches Experimentales sur le Determinisme de la 

 Metamorphose Chez les Lepidopteres," Bull. Biol. Suppl., 24, 1. 



Butenandt, A., and Karlson, P., 1954. "Ueber die Isolierung eines Metamorphose- 

 Hormons der Insekten in kristallisierter Form," Z. Naturforsch. 9b, 389. 



