CHAPTER XXXIII 



THE DUCTLESS GLANDS 



The ductless glands, or endocrine organs, are a group of 

 structures remarkable no less for their function than for their 

 mode of development, and their evolutionary history. The 

 method of pouring out a secretion into the blood-stream 

 instead of leading it away by a duct, is secondary, and some 

 glands which are now ductless doubtless possessed ducts at 

 earlier stages in evolution. Others, comprising the majority 

 of the endocrine organs, were originally not glands at all, but 

 structures which have become useless in the sense that their 

 original function is not or cannot any longer be performed. 

 They have become modified and their functions have changed 

 in a remarkable manner. It is perhaps not without significance 

 that so many of the ductless glands should have such a his- 

 tory of structural and functional transformation. Another 

 peculiarity which applies to several at least of these organs is 

 that in development they arise from two separate rudiments, 

 distinct in manner and place of origin, and even in the germ- 

 layer from which they are formed. 



The method of secreting into the blood-stream carries 

 with it a property which cannot be possessed by glands secreting 

 by means of definite ducts, for the latter can only communicate 

 with definite and restricted spaces in the body, and the effects 

 of such secretions must be only local. On the other hand, 

 the blood circulates all over the body, carrying the endocrine 

 secretions with it. These can therefore affect the body as a 

 whole, and they are of immense importance both during 

 development and during adult life in effecting correlations of 

 the various parts with one another. The ductless glands act 



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