CASPERSSON'S CYTOLOGICAL SYSTEMS 115 



between the duplication of the genes and the presence of nucleic 

 acid." 



Taking the salivary gland nucleus of Drosophila as typical of 

 an intermitotic nucleus, Caspersson advances the view that the 

 euchromatic * regions of a chromosome produce proteins which 

 are rich in tryptophane and tyrosine, whereas the heterochro- 

 matic regions produce proteins which are rich in diamino acids. 



Then, turning to reproduction of cytoplasmic protein, Caspers- 

 son believes that the process commences by formation of nucleo- 

 lar protein within a chromocentre. Thus the chromocentre be- 

 comes distended and the Feulgen-positive material appears as a 

 ring, halo, or crescent contiguous with the newly formed nucleo- 

 lus. The chromocentre in this condition is commonly called 

 the "nucleolus-associated chromatin." There is a very great 

 volume of evidence to indicate that the nucleolus is intimately 

 connected with formation of cytoplasmic protein. So far as is 

 known there are few, if any, exceptions to the rule that cells 

 which are rapidly forming cytoplasmic protein have prominent 

 nucleoli. During the formation of ova in the ovary, in which 

 very large quantities of cytoplasmic protein are formed, one or 

 more nucleoli are usually by far the most striking feature of the 

 nucleus. In developing eggs, nucleoli are far from prominent 

 during the early cleavage phases, which appear to be primarily 

 concerned with securing distributions of the. original yolk mate- 

 rial and the establishment of the early differentiation pattern. 

 But after this new antigens, i.e., new proteins, appear in the 

 embryo; during this phase the nucleoli become prominent. 



According to Caspersson, the nucleolar material is rich in di- 

 amino acids. Although the nucleolus itself is lacking in deoxy 

 nucleic acid, it may absorb light strongly in the region of 2600 

 A.U. and is then supposed to be rich in pentose nucleic acid. The 

 content of pentose nucleic acid is,^ however, very variable. 



It was also found that within the nucleus there is a gradient 

 in concentration of the protein rich in diamino-acid from nucleo- 

 lus to the nuclear membrane. In the cytoplasm there is a high 

 concentration of pentose nucleic acid and of protein adjacent to 



* Euchromatic regions have a low content of deoxy nucleic acid between 

 mitoses, whereas heterochromatic regions retain much deoxy nucleic acid 

 during the intermitotic phase. 



