88 THE VITAL FUNCTIONS 



and is carried on in the separate new organism, or 

 organisms, which is, or are, the offspring. 



The simplest form of reproduction is binary fission 

 such as we saw in the amoeba, the division of the body 

 of the parent into the bodies of the offspring. We 

 do not completely understand the forces which set this 

 process at work. All we can say is that it seems to 

 be ultimately determined by a size limit corresponding 

 with a limit of the surface/bulk ratio, which decreases 

 as the organism grows in size. We know that the 

 surface of a sphere increases as the square of the radius, 

 while its volume increases as the cube of the radius. 

 Thus a unicellular organism, as it grows in size, will 

 have a smaller and smaller surface in proportion to 

 its volume, and this will have an important effect on 

 the- processes going on within the protoplasm. The 

 supply of oxygen, for instance, which has to pass through 

 the surface from the surrounding water will be pro- 

 gressively less for each unit volume of protoplasm 

 the larger the organism becomes. It is probable that 

 some effect of this nature initiates the process of division. 



A large number of unicellular organisms are reproduced 

 by binary fission or by some simple modification of it. 

 But multicellular organisms produce special reproduc- 

 tive cells which are separated from the bodies of the 

 parents and produce new organisms (offspring) by 

 growth and differentiation, which may begin and be 

 carried on for some distance (in the highest organisms 

 to the greatest extent) before complete separation 

 occurs. In Chapter XII we shall consider the relation 

 of these special reproductive cells to the individual 

 unicellular organism which reproduces itself by binary 

 fission. 



Reproduction may also take place, as in most plant 



d.1 



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K 



