INSECT METAMORPHOSIS : AN APPROACH TO THE STUDY OF GROWTH 317 



regions faithfully preserve the cuticular plaques secreted by the in- 

 dividual epidermal cells. 



When mounted and studied under the microscope, the plaques in 

 the new cuticle were found to be much larger than those in the old 

 cuticle. This signifies that they were secreted by epidermal cells which 

 had undergone a corresponding increase in size under the influence of 

 juvenile hormone. Moreover, the plaques in the new cuticle showed 

 various irregularities attributable to crowding and displacement of 

 cells from the single epithelium. 



So for this particular tissue it is clear that juvenile hormone op- 

 poses the mitotic activity that normally occurs in response to ecdyson 

 during the pupal-adult metamorphosis. The picture here presented is a 

 suppression of mitosis and a growth by cell enlargement. 



At this point it is worth recalling that the growth of larval insects, 

 which normally takes place in the presence of endogenous juvenile 

 hormone, is commonly by cell enlargement. Indeed, in the higher Dip- 

 tera and Hymenoptera (cf. Wigglesworth, 1954) the growth of all 

 larval tissues is solely by cell enlargement, the same number of cells 

 being present at the end as at the outset of larval life. 



These various observations combine to suggest that the suppres- 

 sion of mitotic activity may be one means by which juvenile hormone 

 exercises its conservative function. It is not inconceivable that growth 

 by cell enlargement may, in itself, oppose the flow of information from 

 nucleus to cytoplasm— perhaps by preventing the breakdown of the 

 nuclear membrane. 



However, on more mature analysis it is clear that the suppression 

 of mitotic activity can scarcely account for all of the actions of juvenile 

 hormone. For example, the larval growth of insects other than the 

 higher Diptera and Hymenoptera takes place by mitosis as well as by 

 cell enlargement. Furthermore, even in the higher Diptera and Hy- 

 menoptera, the cells of the imaginal discs grow throughout larval life 

 by mitosis rather than cell enlargement, and yet these imaginal cells 

 are no less sensitive to juvenile hormone than the somatic cells grow- 

 ing by cell enlargement. 



Furthermore, if juvenile hormone acts by inhibiting mitosis, it 

 should be possible to mimic the hormone by the use of other antimi- 

 totic agents. In unpublished experiments testing this point. Dr. Willis 

 and I have been unable to duplicate any of the actions of juvenile 

 hormone by injecting pupae with colchicine or a number of other 

 antimitotic agents, including a series of nitrogen mustards. Though 

 defects and various abnormalities were produced in many of the ex- 

 perimental animals, in no case did they have any resemblance to the 

 effects of juvenile hormone. 



