20 



THE ESSENTIALS OF HISTOLOGY 



Structure of the cells. Each cell consists of protoplasm containing 

 a nucleus. The protoplasm may be either clear or granular, or it may 

 have a reticulated appearance. In some kinds of epithelium it is 

 striated. The nucleus is a round or oval vesicle lying in the proto- 

 plasm. Usually there is only one, but there may be two or more in a 

 cell. In the resting condition (i.e. when not undergoing division) the 

 nucleus is composed of a membrane enclosing a clear substance, which 

 is traversed by a network of filaments (fig. 17, a). There may be one 



FIG. 17. EPITHELIUM-CELLS OF SALAMANDER LARVA IN DIFFERENT PHASES 



OF DIVISION. 



or more globular enlargements (nucleoli) on this network. The nucleus 

 is stained more easily by dyes than the protoplasm ; this staining 

 affects chiefly the nucleoli, nuclear filaments, and membrane. 



Division of the cells. The division of a cell is preceded by the 

 division of its nucleus. This, in dividing, passes through a series of 

 remarkable changes (fig. 17), which may thus be briefly summarised: 



1 . The network of filaments of the resting nucleus becomes trans- 



