THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 65 



Experience teaches that a great polymorphism of mitochondria in a preparation 

 of cells of the same type, acinus cells of the pancreas for instance, is usually the 

 result of experimental error before or during fixation. Such preparations should 

 be discarded unless the polymorphism occurs constantly with different fixatives or 

 can be seen in the living condition. 



Another error which frequently creeps in is a variation in mordanting which 

 manifests itself in the subsequent staining reactions, in the interpretation of which 

 the mordanting must always be borne in mind. But perhaps the most important 

 variable is differentiation. Preparations assume an entirely different aspect with 

 the degree of differentiation. This is particularly true where the differentiator itself 

 is colored, as in the case of methyl green. 



Let us bear in mind that not one of these methods is specific for mitochondria. 

 Janus green is the most specific, but it will stain other structures (p. 59) under 

 certain conditions. The iron-hematoxylin method of staining is certainly the least 

 specific of all, because it even colors chromatin in the same way as mitochondria. 

 Neither is the Altmann method specific. There are fewer objections to its fuchsin- 

 methyl green modification and to the Benda method, though neither of them can 

 be completely trusted. 



Finally, a word as to the relative value of observations on fixed and fresh 

 material. It is the fashion now to insinuate that the older methods of fixation and 

 staining are more or less useless. This should be deplored. It may be said that the 

 value of fixation consists in the rapidity of its action. The cell is killed instanta- 

 neously and a condition resembling more or less closely that of the living cell at a 

 definite moment in the course of its normal functional cycle is preserved. On the 

 other hand, in the observation of living tissue, the much-prolonged time factor con- 

 stitutes a serious and unfortunately but little recognized source of error. A half 

 hour spent in studying mammalian tissues teased out in so-called isotonic media 

 gives abundant opportunity for experimental error to creep in. Nevertheless, 

 the study of fresh tissue contributes invaluable information as to the process which 

 is taking place, even though the process can not be regarded as entirely normal. The 

 other great and unique advantage which the observation of living tissue presents 

 is that it jiermits of the experimental modification of the various vital processes 

 going on in the cells. To sum up, the methods of fixation of mitochondria will never 

 be replaced by the observation of living tissues, but they will be greatly supplemented, 

 extended, and aided by it. The two must go hand in hand. 



The essential thing about mitochondrial technique is the necessity of experi- 

 mentation. There is nothing really difficult about it, but it is too much to expect 

 to get good results after the first or second trial. For this reason attempts to shift 

 the responsibility onto the shoulders of untrained technicians almost invariably 

 result in mutual dissatisfaction. 



