1881.] Physiology of the Pepsin- forming Glands. 



21 



Hence we may conclude that the granules consist not of pepsin, but 

 wholly or in part of zymogen. 



The processes in these pepsin-forming glands, then, closely resemble 

 the processes which go on in the pancreatic gland. The cell-proto- 

 plasm stores up zymogen. At the moment of secretion the zymogen 

 is converted into ferment, and probably other organic substances found 

 in the fluid secreted. 



In all the glands, with the exception perhaps of those of the snake, 

 the cells, which diminish somewhat in size as well as in granularity in 

 t Lie first period of digestion, recover more or less completely their 

 normal size and their normal granularity during the latter period of 

 digestion. In other words, at a time when the using up of granules 

 is still proceeding, fresh granules are formed ; and, since the granules 

 result from protoplasmic metabolism, we may conclude, bearing in 

 mind the increase in size of the cells, that the protoplasm is also 

 growing. 



Thus in any gland-cell, during at any rate the greater part of 

 digestion, three processes are going on at the same time, viz., the 

 growth of protoplasm, the formation of zymogen by the protoplasm, 

 the conversion of zymogen into secretory products. There are certain 

 reasons which make it in a high degree probable that these three pro- 

 cesses go on during the whole of the digestive period. Under certain 

 conditions, there is a preliminary increase in the size of the cells and the 

 granules before the normal decrease sets in. When the decrease in the 

 size of the cells does set in, it is not sufficient to allow us to suppose 

 that there is no growth of protoplasm ; the casting out of the granules 

 which have disappeared would, if there were no protoplasmic growth, 

 leave the cell much smaller. The analogy of the change in the 

 mucous salivary glands during secretion affords an instance in which 

 the cell- protoplasm increases steadily during activity. 



Lastly, as regards the granules, it is clear that there might be a 

 fairly rapid formation of granules without any obvious change in the 

 cell, provided the using up of granules went on at an equal rate. 



On the whole, then, it may be, I think, fairly concluded, that, from 

 the beginning to the end of the digestive period,, the three processes 

 mentioned go on. If this be the case, the different appearances of any 

 cell as regards size and granularity depend upon the relative rates 

 with which the three processes proceed at different times. 



When the amount of change which takes place in the gland-cells of 

 different animals during digestion is compared, very wide differences 

 are found. The most striking comparison is perhaps afforded by the 

 glands in Triton tceniatus and those in Triton cristatus. At the fourth 

 hour of digestion, when the cells have apparently secreted approxi- 

 mately proportional amounts of pepsin, i.e., when they have used up 

 an approximately proportional amount of granules, the observable 



