(Monty, et al, 1956). The model of the interphase nucleus and the 

 scheme involving formation of the nucleolus from the chromocenter pro- 

 posed by Caspersson (1950) is based largely on the salivary gland 

 nucleus of Drosophila. Apart from the fact that this type of nucleus is 

 highly specialized in terms of development of the chromosomes, there 

 is little evidence to indicate a close morphological relationship between 

 the nucleolus and chromocenter in such nuclei. Kaufmann (1938) dem- 

 onstrated that the single nucleolus in the salivary gland nucleus of Dro- 

 sophila melanogaster is formed by the fusion of two nucleoli which are 



Chromocenter 



In the nucl ecus-associated chromatin 



appears a nucleous 



Nucleous-associated chromatin 



Main bulk ot the nucleolar 

 subst once 



Figure 4-11. Schematic Diagram Showing Development of the Nucleolus 

 from the Nucleolus-associated Chromatin. (From Caspersson, T. O., 1950. 

 "Cell Growth and Cell Function." W. W. Norton & Co.. Inc., New York, 



N.Y., Fig. 48, p. 104.) 



organized independently in association with the sex chromosomes. He 

 also pointed out that, because of the tendency of the heterochromatic 

 regions of the chromosomes of this species to fuse and form a single 

 large chromocenter, the fusion nucleolus may sometimes come to occupy 

 a position within the chromocenter and thereby simulate a direct associ- 

 ation with this structure. In Drosophila biisehii, the chromocenter is 

 almost entirely absent, the nucleolus in this species apparently being 

 connected to the central regions of the chromosomes by a series of 

 chromatic strands (White, 1954). It is also well known that in many 

 other cell types a nucleolus is present where no chromocenters are visible 

 (Barigozzi, 1950; Darlington, 1947). 



86 / CHAPTER 4 



