CHAPTER XL 



THE UNICELLULAR ORGANISMS. 



It lias been shown in the foregoing pages that the complex 

 body of an adult fern or earthworm, or of any of the higher 

 forms of life, originates from a single cell of microscopic size. 

 This cell — the fertilized ovum or oosphcre — gives rise by divi- 

 sion to new cells which in their turn divide, generation after 

 generation, until a full-grown hody is formed, composed of 

 myriads of cells. But the process of cell-division does not in 

 this case go as far as complete o,^- separation^ and the cells do 

 not acquire a complete individuality. They do, it is true, ac- 

 quire a certain independence of structure and function; and 

 their individual characteristics may even dej)art widely from 

 those of neighboring cells (differentiation). Nevertheless they 

 remain closely united by either material or physiological bonds to 

 form one body. The body is not, however, to be regarded as 

 merely an assemblage of independent individual cells. The hody 

 is the individual ^ its more or less perfect division into cells is 

 only a basis for the physiological division of labor; of which 

 cell-differentiation is the outward exjDression. 



All this is true, howe^'er, only in the higher types. At the 

 bottom of the scale of life there is a vast multitude of forms in 

 which the body consists, not of many cells but of only one, and is 

 therefore comparable in structure not to the adult fern or earth- 

 worm, but to the germ-cells from which these arise. Such forms 

 are known as unicellular organisms, in contradistinction to the 

 Tnulticellular. Like other cells the unicellular organisms multi- 

 ply by division, but division is followed sooner or later by com- 

 plete separation ; the daughter-cells become entirely distinct and 

 independent individuals, and do not remain permanently asso- 

 ciated. In them a true multicellular body, therefore, is never 

 formed ; the cell is the individual^ and the hody is unicelhdar. 



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