Need and Action of Reagents 5 



Others, chief among them Fischer,' offer what, in distinc- 

 tion, may be looked upon more as a physical theory. They hold 

 that fixation is always a phenomenon of precipitation of the 

 fluid and semifluid constituents, and that the subsequent differ- 

 ential action of the stains, etc., depends upon physical rather 

 than chemical differences. This theory admits that a given fix- 

 ing agent may produce a peculiar or characteristic form of pre- 

 cipitative or cumulative action, and therefore the differential 

 action of the reagents following it, in a special method, may 

 depend upon this characteristic action of the fixing agent. 



This idea would lead, and no doubt often correctly, to the 

 belief that structures brought out by any method are often arte- 

 facts, or the product of the treatment rather than normal appear- 

 ances. There is much, however, which goes to show that this 

 is not necessarily the case. The precipitation, if such is true, 

 may occur within the elements of the cell in situ and result in 

 molecular structural differences beyond the domain of the micro- 

 scope. 



Either theory permits of the same result for differentiation. 

 The knowledge of the exact and distinctive action of the 

 reagents, both upon the tissue and upon each other, would be 

 highly advantageous in devising a method for a desired result, 

 but otherwise the important thing for the histologist to 'know 

 is whether structures are. produced in form optically different 

 from the normal structures. To him it makes not so much dif- 

 ference whether the action of a fixing agent or of a special mor- 

 dant is to produce a change in the chemical composition of the 

 tissue by entering into it chemically and forming with it new 

 compounds upon which the special after- treatment for a desired 

 result depends, or whether the fixing agent produces merely a 

 change in the construction of the tissue, the chief histological 

 result of which is a change in its physical properties, making possi- 

 ble the differentiation of special structures by a special method 

 of treatment. In all probability the question, in many cases, is 



' Fischer, Untersuchungen uber den Bau der Cyanophyceen und Bakterien 

 Jena, 1897. Also, Fixirung, Farbung und Bau des Protoplasmas. Jena, 1899. 



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