26 



CYTOLOGICAL TECHNIQUE 



spaces filled 

 with mounting 

 medium 



coagulated 



ground cytoplasm 



precipitate 

 DMA 



Fig. 4. A typical spermatogonium, as seen in a routine micro- 

 scopical preparation {diagrammatic). The cell has been fixed in 

 a coagulant fixative, embedded in paraffin, dyed, and mounted. 



Compare with fig. 1 (p. 4). 



No traces of the mitochondria or lipid droplets remain. The 

 centriole, if present, is intact; the idiozome sometimes but not 

 always persists. The nuclear sap has been coagulated, like the 

 ground cytoplasm. The heterochromatic segments of the chromo- 

 somes are still visible and strongly dyed, but the rest of the 

 DNA has been thrown down in the form of an irregular pre- 

 cipitate scattered here and there on the strands of the coagulum 

 and on the inside of the nuclear membrane. The nucleoli retain 

 their form but are somewhat shrunken. 



The non-coagulant fixatives, especially osmium tetroxide and 

 formaldehyde, cause much less initial change in structure than 

 the coagulants. Indeed, while the cells still lie in these fixatives 



