Fig. 22-11. Diagram showing some of the 

 endocrine glands of an insect. (After 

 Bodenstein.) 



specialized group of neurosecretory cells, 

 situated in the brain, mainly near the cleft in 

 the bilobed cerebral ganglion (Fig. 22-11). 

 The nerve cell bodies then transmit the brain 

 hormone, via their axons, to a pair of storage 

 and distribution centers, the corpora cardiaca 

 (Fig. 22-11). In some cases, the intercerebral 

 gland cells of the pupa may temporarily be- 

 come inactive until the pupa has experienced 

 a period of dormancy (diapause) and has 

 been subjected to chilling. 



Molting, or Ecdysis. This process, by which 

 an insect or other arthropod sheds the exist- 

 ing exoskeleton and develops a new one of 

 larger size, is controlled by GDH, or ecdy- 

 sone. In certain crustaceans at least, ecdy- 

 sone is produced by the nervous system 

 (cerebral ganglion), but is stored in and 

 liberated from the sinus gland, a small mass 

 of tissue embedded in the eyestalk. The evi- 

 dence almost conclusively indicates, more- 

 over, that ecdysone and the growth and 

 differentiation hormone (GDH) are identical 

 compounds. 



CELLULAR ENDOCRINOLOGY 



An ultimate goal, of course, is to under- 



The Endocrine Glands - 417 

 INTERCEREBRAL GLAND CELLS 



CEREBRAL GANGLION 



CONNECTIVE 

 CORPORA CARDIACA 

 CORPUS ALLATUM 



stand how hormones act basically; that is, 

 how they act on the cellular and subcellular 

 level. In other words, one would like to know 

 precisely how a particular hormone may 

 change the metabolism or other cellular 

 mechanism in the target organ and how these 

 changes may be related to the over-all effects 

 upon the body. This type of research, called 

 cellular endocrinology, now constitutes a very 

 active field in which important progress 

 seems imminent. 



In the case of insulin, for example, the re- 

 cent evidence indicates that the hormone has 

 an important effect upon cell permeability. 

 Glucose cannot be metabolized properly in 

 the absence of insulin because the sugar does 

 not enter many of the tissue cells. But still we 

 do not know precisely how insulin modifies 

 the molecular structure of the cell mem- 

 brane. And in the case of thyroxin, the re- 

 cent evidence suggests that this hormone 

 changes the structure of the mitochondria, 

 enhancing their oxidative activity. But again 

 we must ask: How and why? The outlook for 

 further progress is excellent, however. Most 

 of the hormones are now available in pure 

 form and many new techniques of study are 

 being developed. 



1. Slow and enduring coordinations tend to be 

 under hormonal rather than under nervous 

 control. Explain this statement exemplifying 

 the discussion by comparing the flow of saliva 

 with the flow of pancreatic juice. 



TEST QUESTIONS 



2. Make a labeled diagram to show the place- 



ment of the various endocrine glands in man. 

 Which of the glands are purely endocrine 

 in function? 

 Carefully explain how deficiency, replace- 



