REACTIONS OF FIXATIVES WITH TISSUES AND CELLS 8l 



the causes underlying the resuhs he obtained. The facts may be 

 briefly stated thus. The best fixation is often obtained if the fixa- 

 tive substance is dissolved, not in distilled water, but in a solution 

 of an 'indifferent' or non-fixative salt, such as sodium chloride or 

 sulphate. 



Miiller ^^^' ^^^ used sodium sulphate nearly a century ago, in a 

 mixture with potassium dichromate, and this salt is used in making 

 up Zenker ^^^ and Helly ^^* to this day, because these fixatives 

 were based on Miiller's. Heidenhain ^'^^ long ago dissolved 

 mercuric chloride in a solution of sodium chloride, and the same 

 salt is commonly used with formaldehyde. 



The advantage of adding an indifferent salt is shown particu- 

 larly clearly by the cells of marine algae and marine invertebrates. 

 Young ^^^ investigated this matter with the neurones of the cuttle- 

 fish, Sepia officinalis. He cut the stellate ganglion in two and put 

 one half into a solution of a fixative in distilled water, and the 

 other into a solution in sea- water. The two halves were then em- 

 bedded together in paraffin and sectioned. When the fixative was 

 picric acid, chromium trioxide, formaldehyde, osmium tetroxide, 

 or potassium dichromate, the half fixed in the solution in sea- 

 water was greatly superior to the other (fig. 5, b-e, opposite 

 p. 67). It appeared that when the cells were placed in a solution of 

 the fixative in distilled water, they swelled, burst, and then col- 

 lapsed with serious shrinkage and distortion, though this sequence 

 of events was not actually observed; in the presence of sea- water 

 they more or less retained their shape. Fixation by mercuric 

 chloride and acetic acid was, however, scarcely affected by the 

 presence of the salts of sea-water. 



Hertwig ^^^ made a quantitative study of the effect of sea- water 

 on the fixation of the eggs of sea-urchins. Some of his results are 

 shown in fig. 12. It will be seen that when formaldehyde was dis- 

 solved in distilled water, the eggs increased greatly in volume dur- 

 ing fixation, but they did not burst. Increase in volume was slight 

 when sea- water was used as solvent, and the eggs remained 

 smaller than the others throughout dehydration and in xylene. 



The cells of fresh-water and terrestrial organisms are less 

 sensitive to the presence of indifferent salts than those of marine 

 algae and marine invertebrates, but similar effects have been 

 observed in them by Sjovall,^^^ Carleton,^^* and others. For many 

 references to the early literature of the subject, see Schaffer.*^^ 

 When the vertebrate kidney is fixed in formaldehyde dissolved in 



