SEASONAL CHANGES IN INTERSTITIAL CELLS 497 



such cells under higher magnification and as affected by the 

 osmic acid of Meves' fixer, is shown in figure 25. The large 

 pigment globules have evidently been derived from the fatty 

 material with which these cells previously were filled. These 

 large pigment cells are most numerous just at the close of 

 these retrogressive changes. The pressure having been relieved 

 by the enormous decrease in the size of the other cells, these 

 pigment cells are more or less spherical at this stage. Osmic 

 acid still darkens the granules, at least on the surface; but they 

 are very insoluble, being preserved fairly well even in Carnoy's 

 fluid as will be seen in figure 4, which also shows that from the 

 very first the nucleus may be irregular, which is most often the 

 case, though it may in some instances be apparently normal and 

 vesicular as in figure 5. Figure 5 is taken from a section which 

 passes through the cell near the middle plane and indicates that 

 the now-pigmented globules may still retain their peripheral 

 arrangement, leaving a less pigmented area in the center. This 

 possibly represents somewhat of an intermediate stage in the for- 

 mation of the more solid and irregular pigment cells. Here then 

 .we evidently see the source of the pigment cells that have been 

 followed through the preceding stages. The interpretation that 

 they originate from ordinary interstitial cells which undergo a 

 special pigmentarj' degeneration at the time when the rest of the 

 cells lose their fat and become small, is borne out by the absence 

 of these pigment cells in w^oodchucks that are less than one 

 year old and have not passed through this adult retrogression 

 of the interstitial cells. No pigment granules are found in any 

 of the interstitial cells of the twelve animals less than a year 

 old. Fatty granules which are blackened with osmic acid are 

 however present in small quantities in the scanty cytoplasm of 

 some of the interstitial cells of these young animals. Thus a 

 new crop of pigment is produced once a year as fine granules 

 within the ordinary interstitial cells and as larger granules 

 which fill more or less completely certain other interstitial 

 cells, which as a result do not at this time decrease much in 

 size but remain as large pigmented cells for many months or 

 even a year and perhaps a few survive even longer, though ap- 



