yo FIXATION 



engaged in this work has provided himself with the pre-requisite 

 knowledge of the subject of fixation. It has seemed sufficient to 

 put living cells under one of the kinds of microscopes that evade 

 the difficulty of transparency, and simply watch what happens 

 when the fixative is added. It is not always realized that this is a 

 test of preservation, not of fixation. It has already been remarked 

 (p. 28) that the classical work of this kind was done by Strange- 

 ways and Canti,*^^ by dark-ground microscopy. 



The most convenient cells are those that will flatten themselves 

 against a coverslip and adhere firmly to it. The amoeboid meso- 

 dermal cells of various kinds that move outwards from cultured 

 tissue-fragments fulfil these requirements well.*^^' ^*' ^^*' ^^^ Their 

 thinness makes them readily observable by dark-ground or phase- 

 contrast microscopy, and their adherence makes it easy to replace 

 the culture-fluid by a fixative. The replacement can be achieved 

 by quite simple apparatus,^^^ but a special perfusion-chamber has 

 been designed ■^*- that w^ould lend itself well to this kind of work. 



The extreme thinness of cultured amoeboid cells, when 

 stretched over the surface of a coverslip, presents disadvantages. 

 Such cells are far from being representative of cells in general, and 

 their firm attachment and unusual shape prevent them from 

 shrinking, or at any rate from showing clearly that they shrink 

 (apart from the retraction of pseudopodia). Loose cells have their 

 merits, despite the technical difficulties they present. Among those 

 that have been used are salivary corpuscles, ^^^ polymorphs, *^^ 

 cultured neurones,-"*^ and amphibian spermatogonia.^^* 



The results obtained by research of this kind will be incor- 

 porated briefly in chapters 5 and 6. By the common consent of all 

 engaged in it, osmium tetroxide gives the most faithful preserva- 

 tion of the living condition of the cell (fig. 8, A, b). Chromium 

 trioxide may be cited as an example of a fixative that alters the 

 structure considerably (fig. 8, c, d). The pseudopodia are blunted; 

 cytoplasm and nuclear sap coarsely coagulated; lipid droplets 

 fused (probably by the stresses of coagulation); mitochondria 

 destroyed. In dividing cells, however, the chromosomes are 

 rendered clearer than they were in life. 



One would wish to obtain quantitative information about the 

 chemical changes wrought by fixatives on tissues and cells. A 

 piece of tissue may be analysed chemically, as a whole, before and 

 after fixation,^^^ or the fixative solution may be analysed for dis- 

 solved tissue-constituents after fixation, *^^ but the substances 



