THE STRUCTURAL BASIS OF THE BODY 



15 



twofold structure is also found. So 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 CyanophycesB and Bacteria among the 

 latter, no distinct nucleus can be demonstrated. In many of these forms the dimen- 

 sions of the whole organism are too minute to allow of any definite statement being 



W V 



' \ 



e 



FIG. 2. Amceba proteus, an animal consisting of a single naked cell, X280. 



(From SEDGWICK and WILSON'S Biology.) 

 n, the nucleus ; wv, water- vacuoles ; cv, contractile vacuole ; fv, food-vacuole. 



lade as to the presence or absence of nuclear material. In the larger of them, how- 

 ever, the cytoplasm of the cell contains numerous scattered granules which stain with 

 dyes in 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. 



We have defined a cell as a small mass of protoplasm containing a nucleus, 

 ince we shall have to use the term ' protoplasm ' on many occasions in the 

 rse of this .work, we must have a definite conception of what we mean 

 y it. The term is often used by histologists as implying a substance of 

 certain definite chemical and staining characters. When employed by 

 hysiologists it generally implies any material which we can, on a study of its 

 Saviour to changes in its environment, regard as endowed with life. 

 Huxley has defined it as " the physical basis of life." Though it may be con- 

 venient to have a word such as protoplasm signifying simply ' living material,' 

 t is important to remember that there is no such thing as a single substance 

 protoplasm. The reactions of every cell as well as its organisation are the 

 ultant of the molecular structure of the matter of which it is built up. 

 e gross methods of the chemist show him that the composition of the 



