318 CELLS, TISSUES, AND ORGANISMS 



It may be noted, however, that, in contrast to juvenile hormone, 

 colchicine was found to be extremely toxic to insects. Therefore we are 

 unable to state whether mitotic activity was eflFectively suppressed by 

 sublethal concentrations of colchicine. 



Metamorphosis and biological death 



Though we speak of ecdyson as a "growth hormone," it is, in fact, 

 a powerful killer of cells when its action is unopposed by juvenile 

 hormone. For example, during the prepupal period one cannot fail to 

 be impressed by the widespread death of cells in specialized larval 

 tissues. Still later, at the outset of adult development, ecdyson acting 

 in the absence of juvenile hormone promotes another great wave of 

 cell death in specialized pupal tissues. At each stage one can accu- 

 rately predict which cells will develop and which will die. So it is no 

 exaggeration to say that metamorphosis is a tidy blend of birth and 

 death at the cellular level. 



Manifestly these happenings are prerequisite for the two-stage 

 renovation of the insect at the time of metamorphosis. Moreover, it is 

 worth recalling that a larva approaching metamorphosis becomes a 

 closed system without any further intake of food or water. Therefore 

 the pupa must be constructed from the biochemical assets of the larva; 

 the adult, from the assets of the pupa. So, in biochemical terms, meta- 

 morphosis is a reworking of atoms and molecules in an essentially 

 closed system. 



There can be little doubt that the death of specific cells is a part 

 of the "construction manual" for the insect as a whole. The cells that 

 will die have been programmed to do so. Therefore their individual 

 deaths in response to ecdyson represent the decoding and acting out 

 of a fresh, albeit final, bit of genetic information. 



Clearly the biochemical mechanism of biological death is a mat- 

 ter worthy of detailed attention. The intracellular, membrane-limited 

 "lysosomes" are of special interest in this connection. As De Duve 

 (1959 a, b) has shown, these organelles in mammalian liver cells are 

 little short of biological "booby-traps." When activated, they release 

 all the enzymes necessary to take a cell apart. Whether lysosomes are 

 present in insect cells, and their possible role in cell death, are matters 

 which urgently require investigation. 



Juvenile hormone and biological death 



The great waves of cell death are seen only at the time of meta- 

 morphosis; that is, when ecdyson acts in the virtual or complete ab- 

 sence of juvenile hormone. Earlier in the life history, when juvenile 



