IV THE CELL AND THE ORGANISM 73 



organic substance, whose internal equilibrium would pass 

 through various crises and changes in its " development," 

 would finally tend to break up and under certain conditions 

 proceed to divide. This original haphazard division would 

 gradually become stabilised and standardised, so to speak, 

 until cell-division becomes the regular basis not only of all 

 growth but also of all reproductive processes in both plant 

 and animal. 



At first there could have been no essential difference 

 between growth and reproduction of cells. By division 

 one cell was formed from another, and might either remain 

 in association with the old cell, as is the case in all multi- 

 cellular organisms (growth), or it might separate from the 

 old cell and develop on its own as an independent organism 

 (reproduction). This simple division still remains the pro- 

 cess of growth in all organisms without distinction, and it 

 remains the process of reproduction in all unicellular and the 

 lowest forms of multicellular organisms. In less primitive, 

 more developed multicellular organisms the process of repro- 

 duction has, however, become more complex, and has altered 

 to the union or fusion of two specialised cells to form a 

 new cell; and in such cases another scheme, involving a 

 double set of divisions, has taken the place of the simple 

 division. While one of these simply halves the total 

 contents of the old cell as between the two new cells, the 

 other or reduction division separates out the individual ele- 

 ments in the contents so that each of the two new cells 

 has half of these elements (e.g. the chromosomes of the 

 nucleus). This halving is necessary to prevent the continual 

 and cumulative doubling of ceil elements in the repeated 

 reproduction of the same type of organism, and to keep the 

 contents of cells of similar organisms constant. The two 

 cells, thus reduced in all respects to half the original cells, 

 then unite to form the new cell, which has once more the 

 full complement of cell elements. This reduction division in 

 reproduction is common to both plants and animals above 

 the most undeveloped types, and we therefore seem to have 

 some justification for the most remarkable conclusion that 



