ESSAYS 



THE FUNCTION OP THE NUCLEOLUS IN THE LIFE OF 

 THE ANIMAL CELL (J. Bronte Gatenby, B.A., B.Sc, D.PhU. 

 (Oxon), Lecturer in Cytology, University College, London). 



That an animal body is built up of individual cells, just as a brick house is 

 built up of individual bricks, is a fact familiar to most people. The cells 

 which form our bodies are differentiated in various ways for different functions, 

 but all of the cells of the body are similarly equipped with special cell organs. 

 In all cells one finds a nucleus or central body, containing small elements 

 known as chromosomes, which are generally supposed to be the bearers of the 

 factors of heredity. Outside the nucleus, within the main part or body of 

 the cell, called the cytoplasm, one always finds two categories of cell organs — 

 known as the Golgi apparatus and the mitochondria. 



Within the nucleus of the cell, in addition to the chromosomes, there is 

 constantly found one or more small spherical bodies called nucleoli. The 

 latter have been classified under two heads — plasmosomes and karyosomes. 

 The former stain red in a dye like eosin, the latter blue in a dye like haema- 

 toxjdin, so that in the combination of the two stains haematoxylin and eosin, 

 the true plasmosome alone picks up the red stain. 



It can be safely said that all cells at some period of their existence con- 

 tained a nucleolus within their nucleus, and, as a matter of fact, there are very 

 few functional nuclei known in which a nucleolus is not discernible after 

 proper staining methods have been used. The nucleolus of the cell is un- 

 doubtedly a more important cell organ than was probably before thought to 

 be the case, as will be shown here. 



It is a vital issue with geneticists to know the exact degree of relationship 

 between the chromosomes of the nucleus and the nucleolus. The matter 

 is still somewhat difficult to understand, and the problem in some ways un- 

 solved. Nevertheless modern work has thrown a bright light on many of the 

 previously existing darknesses. 



Older cytologists believed that, by using dyes like methyl green, haema- 

 toxj'lin, and carmine, it was possible to distinguish between what was chro- 

 matinic and what was not chromatinic in nature. Nowadays we realise 

 keenly our shortcomings with regard to this. Basophility, or the attraction 

 of some cell substance for a so-called nuclear dye, means very little to modern 

 cytologists ; the whole conception of chromatin is one full of difficulties — 

 difficulties which merely added to by a previously existent idea that chromatin 

 or the substance of heredity and of the chromosomes was some special chemical 

 entity which could always be expected to behave in a special manner under 

 treatment with certain well-known fixing or kilUng substances and basic dyes. 



There is really nothing in the literature of cytology which would enable 

 us to conclude that the nucleolus of the undifferentiated cell was not or is not 

 formed of chromatin ; it cannot be too strongly emphasised that staining 

 tests and digestion tests are not sufficiently satisfactory evidence on this point. 



We can merely undertake a review of the whole matter after having 

 shorn ourselves of the pernicious doctrine that staining tests give valid 

 evidence as to whether a body is or is not cliromatin. 



