GROWTH 89 



be liberated from the corpus cardiacum, but its nature is not known. 

 It certainly acts upon the thoracic gland and causes this to secrete 

 the moulting hormone which is named 'ecdyson'. Ecdyson has been 

 isolated and crystallized in pure form from larvae of Calliphora and 

 from pupae of the silkworm: it is a steroid the chemical structure of 

 which is now known. It exerts its effect directly upon those cells 

 concerned with growth and moulting; it 'activates' them and stimu- 

 lates them to renewed protein synthesis. 



The juvenile hormone, besides its control of metamorphosis, is 

 concerned in the reproductive activity of both sexes (p. 102). It 

 accumulates in considerable quantities in the adult male of the giant 

 silk-moth Hyalophora cecropia, and if the fats are extracted from the 

 abdomen of this moth they appear as a yellow oil rich in juvenile 

 hormone activity. The active substance has been concentrated some 

 50,000 times, but the chemical nature of the active principle is not 

 known. Extracts from many other invertebrates, certain organs of 

 vertebrates (such as the suprarenal glands) some higher plants, 

 bacteria, and yeasts also contain material with juvenile hormone 

 activity. In several instances this material has been proved to be a 

 mixture of the isoprenoid alcohol farnesol and aldehyde farnesal. 

 It may be that the active group in the natural juvenile hormone of 

 insects will prove to be the triple isoprene unit of trans-trans- faniQSol, 

 in suitable combination. The mode of action of the juvenile hormone 

 will be considered in relation with 'polymorphism' (p. 94). 



Histolysis and histogenesis 



In the processes of growth and in the determination of form in 

 insects the effective tissue is the epidermis. There is the utmost varia- 

 tion in the behaviour of the internal organs. Even in the extreme case 

 of metamorphosis in the Diptera, many larval organs (Malpighian 

 tubes, certain muscle groups) are re-modelled without much change 

 to form those of the adult. Other organs undergo extensive break- 

 down (histolysis) and new organs are built up from the embryonic 

 cells of the imaginal discs (histogenesis). There has been much con- 

 troversy about the physiology of this process. It is now clear that 

 when metamorphosis begins certain of the larval cells die and disin- 

 tegrate (autolysis), but the physiology of this process is not known. 



