368 MARGARET R. LEWIS AND WARREN H. LEWIS 



struction of the nucleus, as indicated in figures 17 and 18. The 

 younger nuclei are smaller, darker and more compact and the 

 cells are smaller. The older cells are larger and contain larger 

 nuclei which are less and less deeply stained. In such a series 

 (fig. 17) the number of mitochondria increases from about 40 

 to 150. On the other hand, the old resting cell (k) with a very 

 pale nucleus has only 32. In one series (fig. 18 i, j, k, 1) the 

 number increases from 24 to 140. In figure 18 one of the two 

 daughter cells (c) has 37, the dividing cell (a) has 140, while the 

 two neighboring resting cells (e) and (f) have 39 and 47. Again, 

 in figure 18, the young daughter cells (d, d) have 37 each while 

 older neighboring cells (g, h) have 56 and 58 each. On the other 

 hand, another dividing cell (b) near this same group has but 

 60 mitochondria. 



From such observations one might conclude that there is a 

 gradual increase in the number and in the size of the mito- 

 chondria during the growth period of the daughter cells. The 

 greatest increase both in number and size seems to occur then 

 during the so-called 'resting' period which is in reality a period 

 of growth both for the mitochondria and for the nucleus. On 

 the other hand, while we are unable to determine definitely 

 whether the nmiiber of mitochondria actually increases during 

 the early stage of mitosis there are frequently indications that 

 such cells have more mitochondria than mature cells Cell d 

 (fig. 5) early prophase has 89 while the resting cells a, b, c have 

 47, 51 and 48 mitochondria each. In figure 4 e the dividing 

 cell has 118 while the three neighboring cells have 102, 126 

 and 62 mitochondria each. The two cells with the larger nuclei 

 are probably older resting cells and each has about the same 

 number of mitochondria as in the dividing cell. 



Numerous other specimens seem to show that the dividing 

 cells often have more mitochondria than any of the fullgrown 

 resting cells in the immediate neighborhood. Sometimes this is 

 so marked that there is every indication that the number of the 

 mitochondria in some instances may increase considerably dur- 

 ing mitosis. It seems probable thereforee that mitochondria 

 increase in number both during the resting period and during 



