144 ALEXANDER L. BOUNCE 



into the cytoplasm (see references cited in footnote 80). It should be noted 

 that, since there is apparently no physical union between the amphibian 

 egg-cell type of nucleolus and the chromosome, there is no reason to predict 

 any simple or constant relationship between the number of such nucleoli 

 and the number of chromosomes in the nucleus. 



The egg of the common snail contains two kinds of intranuclear inclusion 

 bodies which might be termed nucleoli but which have different staining 

 properties.^** 



In nuclei of mammalian cells, the situation is more complex, and it is 

 here that the greatest disputes arise as to what should be called nucleoli. 

 No nucleolar migration has apparently been observed in mammalin so- 

 matic cell nuclei, and moreover the number of nuclear inclusion bodies 

 that are commonly termed nucleoli by histologists and cytologists seems 

 to depend upon the degree of polyploidy of the cell, since according to 

 Biesele et al.^^^ a direct relationship exists between the degree of polyploidy 

 and number of nucleoli. Such a situation would be predicted from the cine- 

 photographic studies of Warren Lewis on the nucleoli of rat fibroblasts.^^' 

 Lewis found that the nuclear inclusion bodies of these cells, which are com- 

 monly termed nucleoli, are integral parts of chromosomes, and that they 

 become dispersed during mitosis and reappear in condensed form during 

 interphase when the chromosomes are dispersed. 



Such nuclear inclusion bodies might be termed heterochromatic centers 

 by Schultz and Caspersson,^*^'!*'* but, if such is the case, it would appear 

 that the "true nucleoli" of Caspersson and Schultz are generally not ob- 

 served at all in mammalian somatic cells. In any event, it is such common 

 practice to refer to the Warren Lewis type of nuclear inclusion body as 

 nucleoli that it is very doubtful whether this trend could now be reversed 

 even if it were desirable to do so. It might in fact be preferable to rename 

 the inclusion bodies found in amphibian egg cells, if the question of termi- 

 nology should become so important that a change should be demanded. 

 However, it would seem simpler to retain the word nucleolus as a general 

 term, and speak of two different types of nucleoli, or more than two if this 

 becomes necessary. In this article, the general term nucleolus will be used 

 to designate both the free-floating amphibian egg-cell type of intranuclear 

 inclusion body and the intranuclear inclusion bodies of mammalian somatic 

 cells which are attached by stalks to chromosomes. 



There is one doubtful point concerning nucleoli which should be settled 

 as quickly as possible by the biologists, namely the question of whether 

 any of the microscopically observable intranuclear inclusion bodies found, 



'*' V. Emmel, University of Rochester School of Medicine, personal communication. 

 188 J. J. Biesele, H. Poyner, and T. S. Painter, Univ. Texas Publ. No. 4243 (1942). 

 1" W. H. Lewis, Bull. Johns Hopkins Hosp. 66, 60 (1940). 



