CHAPTER III 



SECRETION 



The last form of response which we shall consider is secretion. 

 The phenomena of secretion have been studied far more from 

 the standpoint of their relation to other functions than with a 

 view to throwing light on the mechanism by which the gland 

 cell elaborates and discharges its specific products of its activity. 

 In vertebrates secretion has been studied especially in relation 

 to the digestive processes (which will be discussed in a later 

 chapter), to the elimination of waste products (renal secretion), 

 heat regulation (sweat glands), and reproduction (the mammary 

 apparatus, oviducal secretions, etc.). Slime-secreting skin 

 glands have also received some attention. But the study of 

 other animal groups opens up an immense variety of secretory 

 mechanisms, some of which are doubtless of considerable 

 bionomic interest as a means of defence or attack. Our know- 

 ledge of a few of these will be briefly indicated by reference to 

 such phenomena as bioluminescence, gas secretion, silk forma- 

 tion, etc. Others such as the secretion of ink in cephalopods, 

 of calcareous matter forming the tubes of sedentary animals or 

 the exoskeletons of mobile forms, etc., can only be mentioned 

 in passing. The importance of mucous glands in relation to 

 digestion will be touched on under that heading. There is a 

 vast field for biochemical research awaiting inquiry in connexion 

 with the comparative physiology of secretion. 



Renal Secretion.— Concerning the mechanism of secretory 

 activity, perhaps the study of the vertebrate kidney has yielded 

 more valuable information than any other secretory organ. 

 The characteristic differentiation of each glandular element of 

 the vertebrate kidney into a capsular portion and " renal 



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