ORGANISATION OF NUCLEOLI 305 



A nucleolar organiser, staining deeply in aceto-carmine, lies next to 

 the place where the nucleolus and the secondary constriction are 

 developed. The organiser lies in chromosome No. 6 which under- 

 went interchange with No. 9 as a result of X-raying. The inter- 

 change break in the chromosome occurred within the organiser and 

 divided it into two parts that were now lying in different chromo- 

 somes. These parts proved to have a different degree of activity. 

 The smaller distal part that was transferred to No. 9 gave a larger 

 nucleolus than the proximal part left in No. 6, in normal circum- 

 stances (Fig. 99). When, however, this more active element was lying 

 in an isolated piece of the nucleus at telophase, it gave a smaller 

 nucleolus. Moreover, the weaker organiser developed a full-sized 

 nucleolus if the stronger organiser in No. 9 had been left out of the 

 nucleus (by non-disjunction at meiosis). It follows that the size of 

 nucleolus developed depends upon the amount of material available 

 in the chromosomes lying around it, and if there are two organisers, 

 on their relative rate of action. When there are two organisers there 

 is competition for the available materials. A further significant 

 observation of McClintock is that the organising capacity can be 

 suppressed by particular genotypic changes in the chromosomes, 

 such as unbalance or loss of parts, even though these parts do not 

 themselves contain specific organisers. The nucleoli then appear 

 as droplets distributed over the chromosomes generally. These 

 droplets may fuse and the chromosomes then appear to be embedded 

 in the nucleolar material. The same result ensues when no organiser 

 is present in a defective nucleus. Cases of loss or gain of a trabant 

 (Matthiola, Philp and Huskins, 1931 ; Rumex, Yamamoto, 1933) 

 require re-examination in the light of these results. 



The competition amongst organisers is shown in another way in 

 hybrids of Crepis (Navashin, 1934). Here a strong organiser from 

 one parent seems to supplant a weaker one from the other. Since 

 the nucleolus stretches the constriction, the weaker nucleolar 

 chromosome fails to develop its characteristic long constriction in 

 the hybrid. McClintock has established a scale of strength for 

 Crepis species in accordance with this view. Competition is 



similarly shown by XY^ -f Y males in Drosophila where the 

 attached arm of the Y does not develop its usual sized nucleolus 



