316 CELLS, TISSUES, AND ORGANISMS 



lished studies). If the corpora allata are excised to remove all traces 

 of hormone from the mature larva, then certain of the larval cells ac- 

 quire an excess of fresh information when acted upon by ecdyson. 

 Consequently these particular cells overshoot the pupal stage and 

 enter at once into a precocious adult differentiation. 



In assays of the corpora allata of the Cecropia silkworm I find that 

 the glands are totally inactive after the pupal molt and throughout the 

 first two-thirds of adult development. Therefore the metamorphosis 

 of the pupa into the adult is distinguished by the fact that it takes 

 place in what appears to be a total absence of juvenile hormone. 



This finding accounts for the extreme sensitivity of the early stage 

 of adult development to the injection of juvenile hormone or the im- 

 plantation of active corpora allata (Williams, 1959). Thus, if the hor- 

 mone is injected into a pupa just prior to the initiation of adult de- 

 velopment, it prevents the formation of the moth. The net result is 

 that the pupa molts into a second pupal stage. With lower doses of 

 hormone, certain cells are inhibited and others are not. Pupae of this 

 type transform into strange creatures showing a mixture of pupal and 

 adult characteristics. 



The critical period for the action of juvenile hormone 



The pupa is maximally sensitive to juvenile hormone at the outset 

 of adult development. If the injection is postponed until the fifth day 

 of the 21 days of adult development, it is already too late for juvenile 

 hormone to prevent the formation of a normal adult moth. Evidently, 

 by the fifth day a full set of genetic instructions for the construction of 

 the moth has already been distributed within the individual cells. 



These findings direct attention to cytological events, including 

 mitotic divisions, which ecdyson induces at the outset of adult de- 

 velopment. Whatever the pertinent cellular or subcellular events may 

 be, we can state that they occur early in adult development, that they 

 show a rapid loss of sensitivity to juvenile hormone, and that, if un- 

 opposed by juvenile hormone, they commit the cells to metamorphosis. 



Juvenile hormone as an antimitotic agent 



In previously unpublished experiments performed in collaboration 

 with Dr. Judith Willis, juvenile hormone was injected into pupae of 

 the Polyphemus silkworm in order to cause the formation of a second 

 pupal stage. Disks of integument were then punched from the abdo- 

 men to include both the new and the old pupal cuticles. Areas of inter- 

 segmental membrane were chosen for study, for these particular 



