PRIMARY fixatives: NON-COAGULANTS II7 



Harde?ting. Formaldehyde hardens strongly. Wetzel, using a 10% 

 solution, obtained a rigidity-figure of about 1700. This is 7 J times 

 the figure for chromium trioxide and is exceeded only by ethanol 

 and acetone. 



Immediate effects on particular constituents of the cell. The cell 

 outhne is often well preserved, but there is a strange tendency for 

 blebs of cytoplasm to separate from the cell. The ground cyto- 

 plasm is not so homogeneously fixed as experiments with gelatine 

 and gelatine/albumin gels would lead one to expect: there is a 

 tendency for a certain amount of granulation to occur. Mito- 

 chondria are preserved, often rather well, though sometimes they 

 become moniliform. When once they have been fixed by form- 

 aldehyde, they are no longer subject to destruction by acetic 

 acid.^^^ Lipid globules usually remain as in life, but a wide variety 

 of these has not been studied. 



The shape and structure of the nucleus are on the whole well 

 preserved, though there is some tendency for the nuclear sap to 

 become granular. The heterochromatic segments of the chromo- 

 somes remain more or less as in life. The nucleolus is less clearly 

 seen than before fixation. 



There can scarcely be any doubt that formaldehyde (with an 

 indifferent salt) preserves the structure of the living cell better 

 than any other primary fixative except osmium tetroxide. 



Methods ofzvashing out. As a general rule, no special washing out 

 is necessary. Tissues may be transferred to water or to ethanol of 

 any grade. 



Dark brown, birefringent crystals are sometimes seen in tissues 

 that are rich in blood, especially spleen. This so-called Tormalin- 

 pigment' appears to arise by the reaction of formaldehyde with 

 the haematin of haemoglobin that has escaped from red blood- 

 corpuscles either before or after death. It is not formed if short 

 fixation in formaldehyde solution is followed by prolonged soaking 

 in 5% mercuric chloride solution. ^^^ Once formed, it can be dis- 

 solved by a 1% solution of potassium hydroxide in 80% ethanol, 

 or by picric acid dissolved in ethanol. ^-^ The chemistry of these 

 processes has not been worked out. 



Effect 071 dyeing. Formaldehyde renders proteins and cytoplasm 

 more acidic (basiphil) than any other fixative, exceeding mercuric 

 chloride in this respect: cytoplasm retains little affinity for acid 

 dyes unless fixation is short. Chromatin is strongly coloured by 

 basic dyes. 



