INSECT METAMORPHOSIS: AN APPROACH TO THE STUDY OF GROWTH 315 



that a conservative mechanism is at work in a larval molt to prevent 

 the flow of fresh genetic information from nucleus to cytoplasm. 



The situation is radically different late in larval life, when the molt 

 in response to ecdyson is accompanied by metamorphosis. Here the 

 old larval cuticle is cast off and a completely different pupal cuticle 

 is synthesized and secreted by the same epidermal cells. Still later, 

 ecdyson is again released to initiate the pupal-adult transformation. 

 The pupal cuticle is molted, and the same epidermal cells now synthe- 

 size an adult cuticle of distinctive type. 



Manifestly the epithelial cells at the time of metamorphosis have 

 at their disposal fresh genetic information prerequisite for the new 

 synthetic acts. This implies a derepression of genes and the passage of 

 new coded information from nucleus to cytoplasm, presumably in the 

 form of ribonucleic acid. Perhaps the day is not far distant when we 

 may speak of larval ribosomes, pupal ribosomes, and adult ribosomes. 



The juvenile hormone 



In the foregoing analysis I have directed attention to the presence 

 in the larval insect of a conservative agency which stabilizes the larva 

 as a larva by opposing the flow of fresh information from nucleus to 

 cytoplasm. This conservative factor has now been successfully ex- 

 tracted and purified (Williams, 1956; Williams et al., 1959). It is called 

 the "juvenile hormone." It is a relatively small, apolar, heat-stable lac- 

 tone ( Williams, unpubhshed studies ) . 



x\s first shown by Wigglesworth (1936), the juvenile hormone is 

 the secretory product of the corpora allata— a pair of tiny endocrine 

 glands in the insect head. In immature larval insects the glands are 

 extremely active in secreting the hormone and thereby preventing 

 metamorphosis. As Bounhiol (1938) and Fukuda (1944) have shown, 

 if one excises the corpora allata and eliminates the source of juvenile 

 hormone, the larva undergoes precocious metamorphosis at the very 

 next molt. 



In the normal course of events, the endocrine activity of the cor- 

 pora allata shows a steady decline during the final larval instar. Con- 

 sequently toward the end of larval life the brakes are released, and the 

 cells throughout the insect can now tap the fresh information prerequi- 

 site for pupation. But in many species one can reapply the brakes by 

 implanting active corpora allata or by injecting juvenile hormone. The 

 net effect is to enforce one or more extra larval instars— a state of 

 affairs which produces insects of giant size (cf. Wigglesworth, 1954). 



In the Cecropia silkworm, pupation occurs in the presence of a 

 low but finite concentration of juvenile hormone (Williams, unpub- 



