THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 85 



of interest when we remember that all our evidence points to the conclusion that 

 mitochondria are concerned in metabohsm. This work of Riddle seems to give us 

 another chance to correlate the amount of mitochondria demonstrated histologically 

 with the results of chemical analysis as well as with variations in physiological 

 behavior. It must be borne in mind that the deutoplasm and yolk are not neces- 

 sarily mitochondrial, but the substances out of which they are built up are phos- 

 pholipins and resemble the mitochondria very closely in some important respects. 



VARIATIONS IN CONSTITUTION. 



In some varieties of cells the constitution of the mitochondria apparently differs, 

 slightly but noticeably, from that of the mitochondria in other cells, though in 

 cells of the same kind their composition is very constant. It was at first noticed, 

 in a casual way, on the examination of preparations, that the mitochondria were 

 occasionally preserved in one kind of cell and were lost, or imperfectly fixed, in 

 others. While it is possible that this may be due to some difference in the cells 

 themselves, it is far more likely that we are really dealing with a true variation in 

 the solubility of the mitochondria. In the staining of fixed tissues, also, one occa- 

 sionally meets with differences in the reactions of the mitochondria to the stain. 

 Similarly, the mitochondria in different varieties of cells color with janus green 

 somewhat differently, some more easily than others. Many investigators have 

 also found that some mitochondria blacken or turn gray with osmic acid, although 

 others do not. But observations such as these are unsatisfactory in the extreme, 

 because they can not be controlled and because there is nothing quantitative about 

 them. 



We owe our first detailed information to Regaud (1910, p. 295), who very 

 carefully compared the solubilities of mitochondria in the different cells of the testis 

 with respect to acetic acid. He found that there is a progressive increase in their 

 resistance to acetic acid as one passes from spermatogonia to spermatozoa. Some 

 years later Nicholson (1916, p. 336) applied the same methods of technique to the 

 central nervous system of the white mouse and found that there also the mito- 

 chondria in certain varieties of cells presented different and characteristic solubili- 

 ties in acetic acid. This will have to be done in other organs and with a large 

 variety of solvents before we shall be able to arrive at even a glimmering of the 

 variations in the constitution of mitochondria which occur in different types of 

 cells. 



Variations in the constitution of mitochondria in the course of histogenesis 

 are often quite pronounced. We have all had the experience that technique 

 which gives good results when apphed to adult tissues is frequently very disap- 

 pointing for embryos. This may be due to factors other than a difference in the 

 mitochondria — to changes in the water-content, for example. Here we have no 

 detailed observations on the mitochondria themselves to fall back on; they are 

 much needed. If we exclude the cases of the transformation of the mitochondria 

 into other substances, I think that we may say that there are no very great differ- 

 ences between the mitochondria in these different stages of histogenesis. Cer- 



