in which nucleoli are formed in association with chromosomes which do 

 not exhibit any such constriction. The specific region of the chromosome 

 active in nucleolar formation is usually designated as the nucleolar zone 

 or nucleolar-organizing region. In those cases in which the secondary 



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Figure 4-10. Electron Micrograph of Nucleolus in Tissue Culture Fibro- 

 blast Cell. The filamentous and granular character of the nucleolus is clearly 

 evident. In the center of the nucleolus is a more homogeneous area corre- 

 sponding to the "pars amorpha." The double-layered nuclear membrane is 

 shown indented at the top of the figure. (Courtesy of Dr. Frangoise Hague- 

 nau. College de France, Paris.) 



constriction is involved, the nucleolar zone may exhibit a morphology 

 distinct from that usually shown by other secondary constrictions of the 

 chromosome complement. For example, in pachytene stages of Zea 

 mays (McClintock, 1934) and certain other plant species, a large, darkly 

 staining body (the nucleolar-organizing body) is associated with the 

 nucleolar zone (Figure 4-9). Many other organisms, however, fail to 



84 / CHAPTER 4 



