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MARGARET R. LEWIS AND WARREN H. LEWIS 



mitosis; perhaps in some more during the resting period; in others 

 more during mitosis and in still others during both periods or 

 only during one. It is very unlikely that one can arrive at a 

 satisfactory solution of such a problem from fixed material, 

 since the bodies we are dealing with are subject to such great 

 changes in number and size during life. The number of mito- 

 chondria is not of much value as an indicator of the total quan- 

 tity of mitochondrial substance. 



e / 



/ c / t/^^-J) 'f 

 / I ,^— — '" ( J- 'i^A i^'ij^' 





Fig. 18 Cells from a 2-day culture of heart from a 5-day chick; X 540 diam. 

 Cell a in anaphase has about 140 mitochondria, while 6 has only about 60, the 

 young daughter cell c has 37, the two daughter cells d, d have 36 each; the older 

 cells e, /, g and h have 39, 47, 58 and 56 mitochondria respectively. In the series 

 i, j, k, I, the .young cell i has only 24, the older cells j and k have 41 and 42, while 

 the mature cell I has 140 mitochondria. 



We have indicated that the daughter cells not only have 

 about one-half the number of mitochrondria found in the mature 

 cells, but that the mitochondria are sometimes smaller. 



Does the increase in number during the growth period come 

 about through division of preexisting mitochondria (a process 

 which frequently takes place) or do mitochondria arise de novo? 

 So far as our observations go, either or both processes may occur. 



