DIGESTION IN THE MOUTH 753 



therefore to speak of the granules in a secreting gland as zymogen 

 granules, i.e. the precursors of zymins or ferments. It is probable 

 that we ought to regard these granules not merely as material pre- 

 cursors of the constituents of the secretion, but as little machines 

 or cell laboratories in which proceed a whole series of chemical and 

 osmotic changes which determine the production of the fully formed 

 secretion directly from the protoplasm and indirectly from the 

 ordinary constituents of the surrounding lymph. 



From an examination of the stained specimens of glands the 

 following stages in the production of secretion have been described : 



(1) In the neighbourhood of the nucleus, and probably with its 



FIG. 329. Cells of pancreas, showing successive stages in activity, A, B, c, D. 

 A, resting; D, discharged gland. (MATHEWS.) 



active co-operation, a differentiation of the protoplasm occurs with 

 the production in most cases of a basophile substance which in 

 hardened specimens generally takes the appearance of filaments. 

 Since these filaments are sometimes regarded as the working pait 

 of the protoplasm, they have been given the name of ergastoplasm 

 (Fig. 329, c and D). 



(2) By a modification of the ergastoplasm granules are formed. After 

 their first appearance these granules undergo gradual modification, as is 

 shown by the fact that the staining reactions of the granules near 

 the base of the cell differ from those of the granules at the free margin. 



(3) When the secretion is excited, the fully formed granules 

 take up water and salts in varying proportions, swell up, and dis- 

 charge their contents into the lumen of the alveolus as the secretion 

 proper to the gland. 



ELECTRICAL CHANGES 



Every localised chemical change in a system permeated by electro- 

 lytes must give rise to electrical differences of potential. It is 



48 



