show any such morphological differentiation in this particular chromo- 

 somal region. It has been claimed that the interphase nucleolus is 

 formed of two different parts: one amorphous, the other filamentous, 

 called the mideoloneme (Estable and Sotelo, 1952). According to this 

 view, the amorphous part undergoes the characteristic cycle of forma- 

 tion at telophase and disappearance at prophase. The nucleoloneme is 

 regarded as a permanent structure which persists throughout mitosis in 

 association with the chromosomes. The presence of filamentous struc- 

 tures within the nucleolus has been revealed by both phase-contrast and 

 electron microscopy (Figure 4-10). It has also been reported that 

 Feulgen-positive granules and threads are sometimes visible in the 

 nucleolus of certain cells. Brachet (1957) suggested that the presence 

 of such Feulgen-positive material could result from penetration of chro- 

 mosome segments into the substance of the nucleolus. The existence of 

 intranucleolar structures corresponding, morphologically, to the so-called 

 nucleoloneme has been known for some time. The question of intra- 

 nucleolar material was discussed over twenty years ago by Kaufmann 

 (1938) in relation to his studies on nucleolar formation in Drosophila 

 salivary gland nuclei. Kaufmann (1938) pointed out that the substance 

 of the nucleolus presents a medium in which elongation and opening out 

 of the constituent parts of the chromosome may occur, and gave evi- 

 dence to indicate that the network of threads and granules sometimes 

 visible within the nucleolus is probably composed of separated chro- 

 monemata with their associated chromomeres. 



The nucleolus in the interphase nucleus of many somatic animal cells 

 is associated with a layer of condensed, Fuelgen-positive material which 

 Thorell (1944) has termed the "nucleolus-associated chromatin." Ac- 

 cording to both Thorell (1944) and Caspersson (1950), the nucleolus 

 is a product of the chromocenter of the interphase nucleus (Figure 

 4-11). Basic proteins (histones) and RNA are supposedly produced 

 within the chromocenter during interphase to form the main bulk of the 

 nucleolus. The subsequent accumulation of nucleolar material expands 

 the chromocenter, with the result that the latter takes on the appearance 

 of a ring of Feulgen-positive material surrounding the nucleolus. This 

 perinucleolar ring of chromatin constitutes the nucleolus-associated chro- 

 matin (Figure 4-11). As already mentioned, present evidence favors the 

 view that nucleoli are formed at telophase in association with specific 

 nucleolar chromosomes and, contrary to the view of Caspersson and 

 Thorell, suggests that the nucleolus-associated chromatin is probably 

 best interpreted as representing parts of these same chromosomes which, 

 in the fixed cell, have collapsed around the periphery of the nucleolus 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 85 



