456 WILLIAM G. VAN DER KLOOT 



inhibitory center of the brain into permanent activity. The neurosecretory 

 cells of the subesophageal gangHon are inhibited, hormone release is prevented, 

 and normal egg development is ensured. 



DISCUSSION AND SUMMARY 



The introduction promised that the term "inhibition" would be extended 

 to cover a wider range of events than is the custom. Whether for example, a 

 neuroendocrine factor producing diapause during embryonic hfe can reason- 

 ably be called an inhibitory hormone is left to others. I think that the evolu- 

 tionary history, if ever uncovered, would show a close relation between nerve 

 cells which inhibit adjacent cells and neurosecretory cells which inhibit at a 

 distance. 



It would be folly to think that the same chemicals are necessarily involved 

 in local and in distant inhibitory effects, aUhough the instance of norepine- 

 phrine shows that this is a possibility. EstabHshing the chemical nature of the 

 inhibitory hormones will be a fascinating task, but the results may not provide 

 answers which can be extended to local transmitters. 



Almost all of the examples deserve study by neurophysiologists. The prob- 

 lems are varied and relate to the central questions of integration in the nervous 

 system. The simplest type of control mechanism is the reflex activation of the 

 inhibitory nerve which leads to the corpora allata of the roaches. Here it 

 would be particularly interesting to study in Leucophaea the activation of the 

 inhibitory center by chemicals released from the developing eggs. 



On a more complicated level are the problems of the long-term control of 

 neuroendocrine systems by the changing external environment. In the Cecropia 

 silkworm the control, though still not completely understood, seems relatively 

 simple. The critical controlling events appear to take place within, instead of 

 between, the brain neurons. However, integrative events between nerve cells 

 are surely used in the long-term regulation of the inhibitory center controlling 

 the optic gland of Octopus and of the X organ neurosecretory cells of the 

 Crustacea. In these animals changes in the environment are dealt with so that 

 the output of a single nerve tract is altered. The attraction to the experimen- 

 talist here is that even though environmental changes are put into the animal 

 through complicated sense organs, the output from the central nervous 

 system converges into a single nerve or tract which could be isolated for 

 recording. 



An even more challenging example of central integration driving a neuro- 

 secretory pathway is found in the silkworm, where the environment during 

 the late stages of embryonic life sets the level of activity in an inhibitory center 

 of the brain. Weeks later, the activity of the inhibitory center determines the 

 output of the neurosecretory cells of the subesophageal ganglion and, by this 

 means, the developmental fate of the next generation. It is hard to think of a 



