CONCLUSIONS FROM EXPERIMENTS 189 



described by various authors. 1 As might be expected, such 

 changes, if they do not proceed beyond a certain limit, are reversible, 

 and recovery occurs rapidly. 



In cells where function is accompanied by extensive accumula- 

 tion and discharge of substances, such, for example, as the gland 

 cells, storage cells, etc., the cycles of activity and morphological 

 change are essentially age cycles, that is to say, the period of loading 

 of the cell is a period of decreasing metabolic activity, of "senes- 

 cence,'' and the period of discharge one of increasing activity, of 

 " rejuvenescence," which makes possible a repetition of the cycle. 

 In such cells the structural changes are often very marked. In the 

 pancreas, for example, the cell which is loaded with the granules 

 which give rise to the secretion presents a very different appearance 

 from the cell after continued stimulation and discharge. 



Figs. 66-68 show different stages in the cyclical changes of the 

 pancreas cells of the toad. Fig. 66 shows the loaded cells ready to 

 secrete when stimulated. The whole outer portion of the cell, 

 i.e., the part next to the duct, is filled with large granules, and 

 cytoplasm appears only near the base about the nucleus. This 

 condition is analogous to that of advanced differentiation in which 

 the cytoplasm has been largely transformed into substances which 

 are inactive or less active. In this loaded condition the pancreas 

 cell is only very slightly active metabolically, and its activity is 

 probably due in large measure to the fact that it does secrete 

 slightly, and so the substance of the granules is being changed and 

 eliminated to some extent, more or less continuously. 



But when stimulated to secretion, the oxygen consumption of 

 the cell increases greatly (Barcroft, '08), the granules rapidly 

 disappear, and the cytoplasmic zone extends from the base of the 

 cells out toward the periphery. Fig. 67 shows four cells in various 

 stages of discharge and Fig. 68, cells after long-continued stimula- 

 tion. In this condition the cell is again capable of a high rate of 

 metabolic activity; if nutrition is present the process of loading 

 occurs once more, and the cell approaches quiescence. 



1 See, for example, Dolley, '13, '14; Hodge, '92, '94; Lugaro, '95; Mann, '95; 

 Pick, '98; Pugnat, 7 oi; Valenza, '96. Further references concerning periodic and 

 other functional changes in structure will be found in these papers. 



