THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 79 



nucleus is lost, but in the senile forms, present in the circulation in man, mito- 

 chondria are entirely absent. In jilants this disapi)earance is associated with the 

 production of chloro])hyll, in animals with the formation of hemoglobin, two sub- 

 stances with strikingi}' similar chemical constitution; in both it is gradual and 

 progressive and runs parallel with an increase in the degree of differentiation and 

 with the age of the cell, general metabolism being diminished and special functions 

 being accentuated (see figs. 4 and 5). 



This is but a single instance of a very widespread phenomenon which attracts 

 attention only in those cells which normally die and are replaced in large numbers, 

 collectively, in the life of the individual, hke the cells of the epidermis. It is not 

 without significance in any theory of senescence. Senescence is now thought to 

 result from excessive differentiation with the heaping-up of relatively inert mate- 

 rials in the cell, which clog the vital processes and proportionally diminish greatly 

 the volume of active cytoplasm. Child pictiu-es these substances as large col- 

 loidal complexes, and Burrows (1917, p. 339) thinks that they are retained by virtue 

 of their relative insolubility. Evidently, we have to deal also with a diminution 

 in the mitochondria, but whether it is the cause or the result of the condition we 

 can not tell. 



AMOUNT OF MITOCHONDRIA IN DIVIDING CELLS. 



It would appear that there must be some increase in mitochondria if the daugh- 

 ter-cells contain the normal amount and if their volume, taken together, exceeds 

 that of the parent cell. Apparently, the increase is progressive and proportional 

 to the volume of the cytoplasm. I have made (1914c, p. 102) a careful study of 

 1,000 dividing cells in cliick embryos and at no stage in the process is it possible to 

 say that the number of mitochondria in the cytoplasm is relatively greater than 

 in the neighboring cells of the same type. This is confirmed by the Lewises 

 (1915, p. 371). 



Usually the mitochondria are distributed in approximately equal amounts to 

 the two daughter-cells, but the distribution is a haphazard one, depending only 

 upon the arrangement of mitochondria in the parent cell. An admirable instance 

 of unequal division is to be found in the cleavage of the ascidian eggs as described 

 by Duesberg (1917, p. 481). In my opinion the great differences in the amount 

 of mitochondria m the tissues of the embryo are determined by the physiological 

 condition (p. 82) of the cells rather than by whether or not many mitochondria 

 had been handed down to them from some remote cell ancestor. 



AMOUNT OF MITOCHONDRIA IN DIFFERENT CELL TYPES. 



Striking variations in the amount of mitochondria obtain, but unhappily 

 there has been but Uttle attempt made to distinguish between absolute and rela- 

 tive variations. The obstacle is particularly exasperating in gland-cells, and this 

 is just where the demand for information is most insistent, because the cycUcal 

 changes in the volume of secreting cells can not be ignored. In all tissues the 

 estimation of mitochondria should be carefully controlled by measurements of 



