96 THE CELL 



The white blood corpuscles, on the other hand, are 

 unspecialised cells, very much hke amoebae (Fig. 5, 

 A, w). Highly speciahsed cells lose the power of 

 multiplying by division as soon as the tissue to which 

 they belong is fully differentiated. In general the 

 power of reproduction by division is the function of 

 an unspeciaUsed cell. 



The Cell Doctrine. — Every multicellular organism begins 

 life as a single cell, which is separated from the body 

 of the parent and by cell division, growth, and (in all 

 but the simplest forms) by differentiation of the products 

 of division gives rise to the body of the adult offspring. 

 In this process of cell division the daughter cells do not 

 separate from one another, as in the binary fission 

 of a unicellular organism, but remain together, and in 

 all the higher forms undergo differentiation into tissues 

 and organs to form the different parts of the body. 



This is the basis of the generalisation — known as the 

 cell doctrine — that organisms consist of ceUs, which are 

 the structural units of the organism, and that the 

 functions of the organism ^s a whole are made up of 

 the sum of the functions of all its cells. Broadly 

 speaking this is true, but certain qualifications have to 

 be made. 



In the first place, in some of the lower forms the 

 body does not consist of distinct cells, though it contains 

 a great many nuclei. Thus in some algse and fungi 

 among the plants the body is composed of a branched 

 tube of cellulose (or similar substance) enclosing a 

 continuous mass of cytoplasm in which are scattered 

 numerous nuclei. As the plant feeds and grows the 

 protoplasm at the tips of the branches increases and 

 the nuclei embedded in it increase in number by division, 

 the wall covering the tip continually having new 



