82 THE MITOCHONDRIAL CONSTIl UENTS OF PROTOPLASM. 



to explain. It is very commonly met with in nerve-cells and in gland-cells (see 

 fig. 6). In the cells of chick embryos I find that a decrease in the mitochondrial 

 content is quite common (figs. 7, 8, and 9), but an increase above the normal 

 very rare. 



SIGNIFICANCE OF VARIATIONS IN THE AMOUNT OF MITOCHONDRIA. 



We can say at once that such variations do mean something, because even if 

 the mitochondria are as inert as iron filings their presence in such variable amounts 

 must surely exercise some influence upon the activity of the cells containing them, 

 and we have good reason to think that they are not chemically inert substances. 



There are at least two series of observations to be explained. In the first 

 place, the association of abundant mitochondria with intense protoplasmic ac- 

 tivity. In cytomorphosis, for example, they are especially numerous in the active 

 stages in the life of the cell and they diminish with senility in both plants and ani- 

 mals. There is a sharp increase in mitochondria with regenerative activity, in 

 compensatory hypertrophy, and in many other conditions. In the second place, 

 there is a distinct reciprocal relationship between the amount of mitochondria and 

 the amount of fat. Where there are few mitochondria there is much fat, and 

 vice versa. Decreased oxidation favors the accumulation of fat and increased 

 oxidation favors its elimination, which suggests at once some connection between 

 the amount of mitochondria and oxidation; and their abundance in the active 

 stages of the hfe of the cell, where protoplasmic respiration is rapid, jioints to the 

 same conclusion. Further evidence of a convincing nature has been detailed 

 elsewhere (p. 134), and it seems probable that normal variations in the amount of 

 mitochondria are in some way dependent on variations in the respiration of the cells 

 containing them. 



