THE STRUCTURAL BASIS OF THE BODY 15 



amoeba, the protoplasm or cytoplasm, often presents further differen- 

 tiation into an outer clear layer and an inner finely granular substance. 

 The latter may contain coarser granules, some of food material, others 

 apparently formed in situ by the surrounding protoplasm, and often 

 small vacuoles (' contractile vacuoles ') which are continually altering 

 their size and serve to keep up a circulation of fluid in the interstices 

 of the cytoplasm. In all cells, whether animal or vegetable, with 



""^'' '' ^ ?'*. :?.?$ '-'^'^'''. v'"-'' " : v \ 



cv 



FIG. 2. Amcdba proteus, an animal consisting of a single naked cell, x 280. (From 



Sedgwick and Wilson's Biology.) 



n, the nucleus ; wv, water- vacuoles ; cv, contractile vacuole ; /w, food- 

 vacuole. 



which we are acquainted, this twofold structure is also found. So 

 that we may define a cell as a small mass of protoplasm containing a 

 nucleus. 



Doubt has often been expressed whether a nucleus is to be regarded as 

 essential to our conception of a cell, In many of the lowest forms of animals 

 and plants, such as the Flagellata among the former and the Cyanophycese 

 and Bacteria among the latter, no distinct nucleus can be demonstrated. In 

 many of these forms the dimensions of the whole organism are too minute to 

 allow of any definite statement being made as to the presence or absence of 

 nuclear material. In the larger of them, however, the cytoplasm of the cell 

 contains numerous scattered granules which stain with dyes in exactly the 

 same way as do the nuclei of the cells of higher animals, and these granules 

 possess the resistance to the action of certain digestive fluids which is typical 

 of nuclei. They may therefore be taken as representing the nucleus in the higher 

 forms. Even in the latter, at certain stages, namely, during the division of 

 the cell, the nucleus breaks up into discrete parts, and there is no reason for 

 believing that such a scattered condition of the nuclear material may not last 

 throughout the whole life of the cell. 



