52 S. S. COHEN 



protein spectrum (Brown et al, 1950). On hydrolysis, many amino acids 

 could be detected. The material was not particularly rich in basic amino 

 acids. Degradation products of the nucleic acids could not be detected. 



8. Nucleoli 



These usually spherical bodies are believed to be expansions of special 

 chromosome regions, which are for the most part heterochromatic m their 

 Feulgen stainabihty. The nucleoH usually disappear during mitosis; possibly 

 they are absorbed into the chromosomal substance. In Fig. 3b the nucleolus 

 has been represented as a coil of linearly arranged particles on a doubled 

 thread, a structure which has been seen as such in electron micrographs of 

 section's of certam nuclei (Novikoff, 1957). Consistent with this representation 

 is the fact that no evidence has ever been obtained for a nucleolar membrane. 

 The absence of such a membrane and the apparent presence and distribution 

 of particles in a plant nucleolus can be seen in the electron micrograph of 



Fig- 6. 



In certain stages of oocyte development, nucleoh may exist m large 



numbers, detached from the chromosomes at the nuclear membrane. Some 

 workers have stated that they pass intact through the membrane, others that 

 their contents are everted into the cytoplasm. According to Caspersson (1950), 

 they appear to be largest and most dense under conditions of extensive 

 protein synthesis. A role in this important metabolic function has been 

 ascribed to the nucleolus, as a unit of transport between chromosomal sites 

 of synthesis and the cytoplasm. No direct evidence supporting this hypo- 

 thesis has yet been presented. 



NucleoH do not take the Feulgen stain, although they do absorb ultra- 

 violet hght. Since this absorption is diminished after treatment with ribo- 

 nuclease, they are considered to contain RNA. Caspersson (1950) had inter- 

 preted spectrophotometric evidence to imply the presence of histones ui the 

 nucleolus. In the nucleoh of the starfish oocyte (Vmcent, 1952), it was not 

 possible to demonstrate the presence of basic protein. On the other hand, the 

 isolated nucleoh of liver cell are reported to contain 22 % of histone, or a 

 protein extractable in cold 0.1 N HCl and precipitable with ammonia (Monty 

 et al, 1956). However, the significant DNA content of these preparations 

 suggests the presence of nucleohistone as a result of contanmiation by 

 chromosomal fragments. 



The isolated nucleoli of the starfish oocytes (Vincent, 1952) were spherical 

 refractile bodies containing one or more vacuoles. They swelled in alkah and 

 were broken down by trypsm. They were sohd or semisohd and contained 

 30 % protem and 2-5 % ENA. The composition of the latter was different 

 from the ENA of cytoplasmic granules m that the former contained more 

 guanine and less uracil. Their enzyme content wall be discussed below. 



