NEURO-ENDOCRINE SYSTEMS OF INVERTEBRATES 455 



Inhibition Between Generations 



The final example shows that inhibitory hormones from neurosecretory cells 

 may be used between individuals as well as within a single body. In some 

 races of the silkworm. Bombyx nwri, the females can lay eggs of either of two 

 types: eggs which promptly go through embryonic development or eggs 

 which enter diapause. For simplicity, the moths laying diapausing eggs will 

 be called "diapause" females, those laying normal eggs will be called "non- 

 diapause" females (Lees, 1955). Whether a female becomes "diapause" or 

 "non-diapause" is determined by the environment in which the animal is 

 reared. If the silkworms are raised in the dark, the mature females are "non- 

 diapause", if reared in the hght, they are "diapause". Surprisingly, in making 

 this determination the environment is most influential during the late stages 

 of embryonic hfe, before the sense organs are exposed to the outside world 

 (Kogure, quoted by Lees, 1955). The fate of the next generation can be 

 decided before the parents have hatched from the egg. 



The eggs enter diapause if they have incorporated a hormone from the 

 maternal blood (Fukuda, 1951, 1952; Hasegawa, 1952). The hormone comes 

 from the subesophageal ganglion, which contains neurosecretory cells. The 

 subesophageal ganglion of both "diapause" and "non-diapause" females 

 can produce the hormone. This is shown by experiments in which subeso- 

 phageal gangha from either "diapause" or "non-diapause" females are 

 isolated and implanted into "non-diapause" hosts. The eggs laid by the hosts 

 enter diapause. 



Similarly, when the connectives between the brain and the subesophageal 

 ganglion of a "non-diapause" female are severed, the operated animal lays 

 diapausing eggs (Fukuda, 1953). The conclusion is that the neurosecretory 

 cells of the subesophageal gangha of the "non-diapause" females are inhibited 

 by the brain (Fig. 7). Darkness in the early life of the female acts to set the 



BRAIN 



I 

 I 



NEUROSECRETORY CELLS OF 

 THE SUBESOPHAGEAL GANGLION 



hormone 



I I 



I I 



I I 



J 1/ 



EGGS 



(promoles diopause) 



Fig. 7. A diagrammatic summary of the control of egg diapause in the silkworm 

 (Fukuda, 1951, 1952, 1953; Hasegawa, 1952). 



> inhibitory nerve; ziz^ inhibitory hormone. 



