134 A. D. LEES 



studied mainly in the silkworm Bombyx, involves a different 

 combination of endocrine organs, a particularly important 

 centre being the suboesophageal ganglion with its associated 

 neurosecretory cells. Pupae and moths, determined by their 

 previous photoperiodic treatment to be non-diapause egg- 

 producers, can readily be induced to lay diapause eggs by 

 implanting ganglia from diapause egg-producers^. The release 

 of the 'diapause hormone' from the suboesophageal ganghon is 

 considerably modified by the brain, which in this instance 

 apparently acts through the nervous paths in the circum- 

 oesophageal commissures. It can either promote or inhibit the 

 synthesis and release of the hormone in accordance with the 

 previous photoperiodic treatment^. The corpus allatum also 

 seems to play some part in diapause regulation, its effect being 

 antagonistic to that of the suboesophageal ganglion. But here 

 again a general control of its activity seems to be exercised by 

 the brain^. 



Suboesophageal ganglia that are active in causing Bombyx to 

 lay diapause eggs have also been obtained from certain species 

 with a pupal diapause. Since the 'diapause hormone' is not 

 known to be concerned in controlling pupal diapause, it seems 

 likely that this secretion is utilized for other physiological 

 functions. In this context it is worth recaUins that the cockroach 

 suboesophageal ganglion secretes the neurohormone responsible 

 for initiating and maintaining the diurnal rhythm of activity^. It 

 remains to be decided whether these substances have anything 

 in common. 



Although the maternal endocrine systems which control the 

 production of the two egg types in Bombyx have been the 

 subject of detailed examination, the actual mechanism which 

 leads to the growth failure in the embryo is still obscure. There 

 is no reason to suppose that the 'diapause hormone' from the 

 suboesophageal ganglion is passed into the egg and acts as an 

 inhibitor. When diapausing embryos of Bombyx are dissected 

 out of the tgg and are suspended in a hanging drop side by side 

 with a non-diapause embryo, the growth of the former is 



