44 CYTOLOGICAL TECHNIQUE 



component of fixative mixtures intended for use in studies of 

 glycogen. 



Rate of penetration. Fast {K = 3-6). 



Shrinkage or swelling. Swells gelatine-albumin gel considerably, 

 but leaves the volume of liver almost unchanged. 



Hardening. Hardens strongly. 



Method of washing out. No special washing out is usually 

 necessary, since formaldehyde is very soluble in water and in 

 ethanol. 



Effect on the appearance of cells in microscopical preparations. 

 The form of cells is rather well preserved, though there is a ten- 

 dency for little blebs of cytoplasm to be nipped off from those 

 directly exposed to the fixative (that is, not protected by overlying 

 cells). Cytoplasm is rendered very finely granular. Mitochondria 

 and lipid droplets are generally well preserved. The nucleus 

 remains remarkably lifelike. 



In paraffin sections there is considerable distortion. Cell- 

 aggregates are separated from one another by wide artificial 

 spaces; cytoplasm is shrunk towards nuclei. Mitochondria are 

 sometimes retained; most lipid globules have been dissolved 

 away. The interphase nucleus retains a nearly lifelike appear- 

 ance, but mitotic and meiotic chromosomes are not well fixed. 



Formaldehyde leaves proteins in a state in which they readily 

 take up basic dyes (see p. 90). This follows from the fact that their 

 acidic groups are unaffected by this fixative. Their basic groups, 

 on the contrary, are to a large extent blocked, and the affinity for 

 acid dyes is therefore greatly reduced. If the protein were allowed 

 to take up all the formaldehyde with which it was capable of 

 combining, all affinity for acid dyes would presumably be lost. In 

 practice, however, this stage is not reached,- because the reaction 

 between protein and formaldehyde proceeds slowly. If soluble 

 proteins or polypeptides be allowed to react with excess of form- 

 aldehyde for 8 hours at 70"" C, they take up about one-half of the 

 total amount that they are capable of binding, and the reaction is 

 still incomplete after 24 hours. '^^ The binding of formaldehyde is 

 of course even slower at room temperature. 



Compatibility with other fixatives. Compatible with ethanol, 



