THE STRUCTURE OF THE CELL 5 



confers polarity on the cell, which is radially symmetrical about a 

 linear axis (though there is no precise symmetry of the flat sides). 

 The eccentricity of the nucleus leaves the bulk of the cytoplasm 

 at one end of the cell, and the idiozome lies near the middle 

 of this cytoplasmic mass. The nucleus of spermatogonia is usually 

 ovoid or spherical, but sometimes concave on the side towards 

 the idiozome. This concavity towards the idiozome is not unusual 

 in various cells, especially certain leucocytes. The nucleus is 

 bounded by a distinct membrane, but this is too thin to show any 

 structure with the light-microscope. 



The nucleus contains spherical or subspherical bodies called 

 nucleoli. These commonly appear after mitosis, close up against 

 certain particular parts of particular chromosomes. Since the 

 latter are present in the form of two equal sets, the nucleoli are 

 usually present at first in equal numbers; but they have a ten- 

 dency to fuse, and many cells contain only one. The nucleoli are 

 particularly dense, heavy objects.^ ^ When a cell disintegrates 

 after death, the nucleoli and lipid droplets are distinguishable 

 after everything else has disappeared. Nucleoli contain basic 

 proteins and ribonucleic acid (RNA), and also, apparently, some 

 phospholipid. They are usually largest in cells that are actively 

 synthesizing protein in the cytoplasm, but no satisfactory explan- 

 ation of this fact has been produced. 



Our knowledge of the structure of the part of the nucleus not 

 occupied by the nucleoli is unfortunately meagre. The electron- 

 microscope has given little help towards the solution of this 

 problem. Some authorities consider that during the late stages of 

 telophase the chromosomes swell until they touch one another; 

 others think that they spin out as thin threads that are separated 

 from one another by a continuous fluid or weak gel, the nuclear 

 sap. It is probable that both kinds of nuclei exist, but the weight 

 of evidence seems to suggest that in most cells the chromosomes 

 are largely in the form of very long, thin threads, not distinctly 

 visible with the light-microscope and not colourable by the dyes 

 that attach themselves so intensely to the mitotic chromosomes. 

 The whole of each chromosome, however, is not present in this 

 form, for certain particular regions, called the heterochromatic 



