360 MARGARET R. LEWIS AND WARREN H. LEWIS 
lected in several very large granules (fig. 9). In certain patho- 
logical conditions Barratt ('13) has found that the mitochondria 
become abnormally large and stain clearly. 
Number of mitochondria 
The number of mitochondria varies greatly in cells of the 
same kind in the same preparation (fig. 12) and in different 
preparations (figs. 4, 5, 12). Numerous counts of the mito- 
chondria in the same kind of cells in the same preparation show 
that there is no one number of mitochondria peculiar to any 
one kind of cell or to any one stage in the development of the 
cell. The number of the mitochondria appear to decrease and 
to increase under various conditions. This may result from 
fusion or division of the mitochondria without much change in 
the quantity of mitochondrial substance; or this may be accom- 
panied by a corresponding increase or decrease in the amount 
of mitochondrial substance, independent of any fusion or divi- 
sion of the already existing mitochondria. This would indicate 
that some of the mitochondria may at times entirely disappear 
and that possibly new ones may arise de novo in the cytoplasm. 
Sometimes most of the cells in a growth undergo such changes. 
When observed on one day they may have rather few mito- 
chondria, while on the following day most of the cells may 
contain a marked increase in the number of mitochondria, or 
the opposite phenomenon may take place. This may or may 
not be accompanied by a corresponding change in the quantity 
of mitochondrial substance. Prolonged action of heat causes 
a decrease in the size and number of the mitochondria, and 
it is hoped that further experimental work will determine what 
conditions cause such changes in the ordinary cultures. 
Quantity of mitochondria 
By the quantity of mitochondria we mean the total mass 
of the mitochondrial substance within a cell. This can only 
be roughly estimated, as some cells with many very small mito- 
chondria have a smaller quantity of mitochondrial substance 
than others with fewer but larger mitochondria. However, in 
