II. A. SCIINKIDKRMAN $1 



These results do not tell us the nature of the synthetic reaction in the insect's 

 brain, nor of the breakdown reaction. However, they do indicate that low 

 temperature renders the brain competent to secrete the brain hormone by 

 slowing down some breakdown reaction within the brain, thereby enabling the 

 brain to accumulate a substance which it synthesizes via oxidative metabolism. 

 This substance is necessary for the production of the brain hormone or is, 

 perhaps, itself the brain hormone. Possibly the substance synthesized is the 

 cholinergic material which \'an der Kloot (43) reports increases in the Cecropia 

 pupal brain after chilling; and perhaps the breakdown reaction is the hydrolysis 

 of this material. 



Recently Andrewartha (3), and Andrewartha and Birch, (4) proposed that 

 low temperature acted by rendering intractable food reserves laid down in the 

 fat body or egg yolk available to the insect, and that this was a necessary step 

 in triggering neurosecretion and represented the action of low temperature in 

 terminating diapause. The experimental results just presented are not consist- 

 ent with this suggestion, since the effects of chilling are reversed by warming. 



A more direct approach to the action of low temperature on the brain would 

 be to chill brains in vitro, and unequivocally demonstrate a direct action of low 

 temperature on the brain itself. This prospect is now feasible, for we have re- 

 cently succeeded in maintaining isolated brains of pupal silkworms alive in 

 hanging drop preparations at 5°C for as long as 2 months, using a modification 

 of the tissue culture medium devised by Morgan, JMorton and Parker (23). 

 Whether or not brains chilled in this manner are rendered competent for neuro- 

 secretion remains to be seen. 



Let us now turn to the final question: the mechanism of action of PGH. 

 In all insects that have thus far been studied, PGH triggers cell enlargement, 

 mitosis and secretion of a new cuticle. Although its action in eggs and embryos 

 is yet to be demonstrated, it very likely is involved in embryogenesis (cf. 49). 

 Exposing a diapausing insect to the hormone effects the immediate termination 

 of diapause and initiation of development. The biochemical mechanism whereby 

 PGH triggers this complex biological end-result has been examined in greatest 

 detail in the Cecropia silkworm by comparing diapausing pupae with develop- 

 ing adults whose tissues have been exposed to PGH. 



The diapausing pupa exhibits two striking physiological characteristics; 

 namely, the complete absence of cell division and an exceedingly low metabolic 

 rate (12 )ul/gm/hr. at 25°C). Not only is mitosis normally absent, but the dia- 

 pausing cells do not divide even when repairing extensive injury. Thus, although 

 pupae heal completely following excision of as much as one-fourth of their 

 epidermis, no mitosis occurs. Healing is accomplished solely by cell enlargement 

 and migration. In the absence of adequate concentration of PGH the pupa 

 spreads itself thin to heal its wounds (39). 



The exceedingly low metabolism of the diapausing pupa (less than V^fo that 

 of the mature larva and I-20 that of the adult moth), has led to the frequent 



