14 GENERAL ZOOLOGY 



a logical conclusion that united the cell theory and the protoplasm 

 doctrine, and provided a material basis for a scientific theory of 

 life. Protoplasm is a living substance and this substance has a 

 cellular structure. 



A cell may be defined as a small mass of protoplasm containing 

 one or more nuclei. The cytoplasm or cytosome forms the body 

 of the cell in which the nucleus is embedded. This means that 

 the protoplasm of the cell is made up of two regions, nuclear and 

 cytoplasmic. These two regions are not merely topographical 

 distinctions, but they actually mark the boundaries of two 

 different kinds of protoplasm which have distinct chemical 

 attributes. It also happens that under certain conditions the 

 boundaries between cells are not sharply marked off, forming 

 what is known as syncytial tissue structure; therefore, the 

 significance of cells does not depend so much upon their morpho- 

 logical distinctness as upon the fact that each cellular region is 

 made up of a nuclear and a cytoplasmic component. The cell is 

 something more than a building block with sharply defined 

 boundaries. It is a functional unit which functions with or 

 without cell boundaries. 



Cells do not conform to a standard shape. Free cells, such as 

 egg cells, particularly those deposited in water, are often spherical 

 in form. Tissue cells generally, owing to the pressure of the 

 surrounding cells, take on some other form. Under conditions of 

 relatively uniform pressure, as in the liver, cells are poly- 

 hedrons with approximately equal faces. At the outer surface of 

 the body, cells tend to become flattened and thin, as in the surface 

 layer of the frog's epidermis. Nerve cells have a round or 

 irregular region in which the nucleus is located and from which 

 slender processes extend, in some cases for several feet. Muscle 

 cells are usually spindle-shaped. Pigment cells have irregular 

 branching processes extending in all directions. In general some 

 correlation exists between the shape of the cells and its function 

 (Figs. 1, 2, and 3). 



The nucleus is usually a centrally located body whose shape 

 conforms with the general shape of the cell. Thus, in a spherical 

 cell the nucleus is rounded, whereas in a columnar cell the 

 nucleus is elongated in the main axis of the cell. In the tissue 

 cells of the metazoan animals, as a rule, there is but one nucleus 

 in a single cell. In syncytial tissue, where cell boundaries are 



