ride produce strong cross linkages between protein molecules and are 

 components of such well-known fixatives as Zenker's, Helly's, Bouin's, 

 Flemming's, and Regaud's (Gray, 1952; Gatenby and Beams, 1950, 

 "Microtomist's Vade-Mecum"). Fixation with formaldehyde is believed 

 to occur in two steps: (1 ) the combination of formaldehyde with chem- 



(a) 



(b) 



Figure 11-2. Schematic Representation of the Molecular Organization 

 of Protoplasm Resulting from Good (above) and Bad (below) Fixation. In 

 good fixation cross linkages between molecules are produced without ex- 

 treme alteration of the molecular organization. In bad fixation the numerous 

 cross linkages produced result in separation of a coarse coacervate from a 

 liquid phase. (From Wolman, M., 1955. "Problems of Fixation in Cytology, 

 Histology, and Histochemistry," Intern. Rev. CytoL, 4, Figs. 3B and 3C, 

 p. 95.) 



ical groupings of proteins to form addition complexes, and (2) the reac- 

 tion of these addition complexes with free molecular groupings of other 

 proteins to form methyleiie ( — CHo — ) bridges. Chemical groups which 

 may form addition complexes with formaldehyde are the amino, car- 

 boxyl, and indole groups of proteins, to mention only a few. The two-step 

 reaction of formaldehyde with the amino groups of protein may be rep- 

 resented as follows: 



212 / CHAPTER 11 



