206 X. H. COWDRY. 



temperature and in the refrigerator (8 to 12 C.); in bright light 

 and in darkness without bringing about any noticeable change in 

 their mitochondria. Some fixatives are rather unstable and 

 cannot be kept, even for a few hours, in bright sunlight or in a 

 warm place. Under these conditions Regaud's formalin and 

 bichromate mixture, for example, undergoes a rapid change 

 characterized by a darkening in color, but its action does not 

 seem to be impaired, though, to be on the safe side, one should 

 avoid it. On the other hand, mechanical manipulation of the 

 tissue before fixation often causes very confusing alterations in 

 the mitochondria especially in the softer animal tissues. 



The distribution of mitochondria within the cell is not altered 

 appreciably in either plants or animals by the technique used, 

 except in instances where mitochondria are present with different 

 solubility, some being preserved and others being destroyed. 

 This is of common occurrence in the pea radicle where with 

 some fixatives, filaments only appear; the granules being ob- 

 literated so that the apparent distribution is altered. 



The technique makes a very great difference in the number 

 of mitochondria in the preparation. Some ingredients of the 

 fixative are particularly likely to destroy them, like the acetic 

 acid already mentioned; so that in the study of unfamiliar 

 tissues we must assure ourselves that the technique is adapted 

 to show all the mitochondria present. 



The shape of mitochondria is so easily modified by the fixative 

 that we must be on our guard here also, especially in plant cells. 

 That is to say, we frequently meet with a fragmentation of mito- 

 chondrial filaments, the filaments of the living cell appearing 

 in the form of rows of granules in the fixed preparation. The 

 reverse change never takes place, for filaments are never formed 

 through a coalescence of granules, under the influence of the 

 fixative. Poor penetration of the fixative in the deeper layers 

 of the tissue often causes the mitochondria to lose their char- 

 acteristic form and to swell up into large spherules or vesicles. 

 Similarly if a film on the surface of the tissue is allowed to dry, 

 before fixation, the mitochondria in it will be profoundly modified. 



The size of mitochondria is also subject to some modification, 

 as shown by the comparison of mitochondria in living cells with 



