chemical groups are associated with the surface of the proteins and the 

 side chains connecting different proteins. These molecular groupings 

 serve to connect protein molecules with each other and with lipid and 

 carbohydrate molecules. The water of protoplasm is either bound to 

 ionized groups of proteins, exists free, or is associated with mobile 

 molecules. Assuming that this concept of the structure of protoplasm 

 is essentially correct, fixation would involve the formation of new cross 



Figure 11-1, Schematic Representation of the Molecular Organization 

 of Protoplasm in the Native State. The thick, wavy chains represent protein 

 molecules — the fibrous ones being linear, the curved ones globular. The 

 thick, straight chains represent lipid molecules and fatty acid radicals. The 

 dots represent water molecules and crystalloids. Intermolecular linkages are 

 shown as dashed lines; intramolecular linkages as solid lines. (From Wolman, 

 M., 1955. "Problems of Fixation in Cytology, Histology, and Histochemis- 

 try," Intern. Rev. Cytol., 4, Fig. 1, p. 89.) 



linkages between protein molecules, with the result that new forces of 

 attraction would be established. If the new linkages developed in this 

 way produce excessive clumping of proteins, the result is displacement 

 and distortion of protein structures in the cell and, hence, "poor" fixation 

 (Figure 11-2 (b)). On this same basis, a "good" fixative would be one 

 which leads to the formation of many cross linkages between proteins, 

 none of which set up attraction forces of sufficient magnitude to disturb 

 radically the spatial relations already existing between the proteins of 

 the cell (Figure ll-2(a)). 



Fixing agents such as formaldehyde, dichromate, and mercuric chlo- 



SURVEY OF CYTOLOGICAL TECHNIQUES / 211 



