. Cytology. Cytology. (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 melanogast
. Cytology. Cytology. (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 ; W. W. Norton & Inc., New York, , 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the o
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