78 



THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 



VII. AMOUNT. 



AMOUNT OF MITOCHONDRIA IN PHYLOGENY. 



What is true in the case of morphology holds also here. In man, as an 

 example of a multicellular organism, with great division of la}-)or among his cells, 

 variations in amount of mitochondria occur because some cells are best fitted to 

 l^erform their duties with much and others with little. From our general informa- 

 tion — we have no specific measurements — we can not say that the protoplasm of 

 higher animals differs from that of the lower ones in the amount of contained mito- 

 chondria. Neither is there any noticeable difference in amount between animals 

 and i)lants. Their ai)parent absence in the myxomycetes, schizomycetes, and most 

 of the algae has already been referred to on i)age 72. 



AMOUNT OF MITOCHONDRIA IN ONTOGENY. 



No definite measurements have been made. It may be said, however, that 

 in the very young embryo the cells usually (p. 81) contain approximately the 

 same amount of mitochondria. As development proceeds toward maturity the 

 different tissues become specialized and distinctive differences in the amount of 

 mitochondria often become apparent. It is possible, but improbable, that there 

 is any noticeable difference in the number of mitochondria in actively functioning 

 cells of young and old animals, but this contingency should be borne in mind until 

 decisive information is forthcoming. There is no reason to believe that mito- 

 chondria vary in amount with sex. 



AMOUNT OF MITOCHONDRIA IN CYTOMORPHOSIS (SENESCENCE). 



Mitochondria progressively decrease in number and finally disappear entirely 

 in the later stages of the life of the ceU. 



iiii\ 





Fig. 4. — Meristem and parenchyma cells of the bean, show- 

 ing the progressive disappearance of mitochondria and the 

 formation of plastids containing chlorophyll. (After 

 Guilliermond, 1912, modified.) 



Fig. .5. — Erythroblast, megaloblast, normoblast, and erythro- 

 cj-tes from bone marrow of a rabbit stained supravitally 

 with janus green, showing the parallelism between the 

 disappearance of mitochondria and the appearance of 

 hemoglobin in the form of a diffuse deposit. (After N. H. 



Cowdry, 1917.) 



I refer to the decrease in number of mitochondria in plant cells which runs 

 parallel to the formation of chloroplastids. It is said that when the plasts are 

 fully formed few if any mitochondria remain (Guilliermond, 1912a, pi. 17), and 

 these are mature and highly differentiated cells. In animals there is a similar 

 disappearance of mitochondria in the life cycle of red blood-cells. In the young, 

 nucleated forms, as they occur in the bone marrow, they are very abundant; but 

 they become less and less so as the cell differentiates. A few persist after the 



