ORIGIN AND DEVELOPMENT 39 



may be in small portions, like the well-known 

 amoeba found in stagnant pools. One of its 

 most remarkable characteristics is that of 

 free movement, by which it may change its 

 shape from time to time. It has the power also 

 of absorbing organic matter from the medium 

 in which it lives, and of converting this into 

 protoplasm. It breathes : using up oxygen 

 and producing carbonic acid. 



Protoplasm is always a constituent of 

 living cells. A cell may consist simply of a 

 bit of protoplasm, or it may consist of proto- 

 plasm having embedded in it a minute more 

 or less globular body known as a nucleus, 

 or it may have a thin envelope surrounding it 

 called the cell wall. A typical cell, therefore, 

 consists of a cell-wall, the protoplasmic 

 contents of the cell, and a nucleus. It can be 

 shown that the cell substance (or cytoplasm), 

 and also the nucleus, often show a network of 

 very fine fibres or a coiled fibre, and that the 

 contents are not structureless, as was at 

 one time supposed. The minute fibres, or 

 portions of fibres, found in the nucleus have 

 a special significance. These take up certain 



