

#^ 





"^H 



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Figure 4-9. Photomicrograph of Microsporocyte of Zea mays in the Mid- 

 prophase of 1st Meiotic Division. The nucleolus is shown in close association 

 with the dark-staining nucleolar-organizing body of chromosomes 6. Both 

 chromosomes are in close opposition along their length, and possess a 

 terminal satellite which is separated from the nucleolar-organizing body by a 

 thin, faint-staining thread. (From McClintock, B., 1934. "The Relation of a 

 Particular Chromosomal Element in the Development of the Nucleoli in 

 Zea mays," Zeits. fiir Zellforsch. u. Mik. Anat., 21, Fig. 1, p. 297. Courtesy 

 of Dr. Barbara McClintock, Carnegie Institute of Washington.) 



in association with each of the two homologous chromosomes often may 

 fuse with the other to form a single fusion nucleolus in interphase. There 

 are diploid species known, however, which have more than one pair of 

 chromosomes which function in nucleolar formation. The region of the 

 nucleolar chromosome most frequently involved in the production of 

 the nucleolus is the secondary constriction, though organisms are known 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 83 



