THE SIMPLER ORGANISMS 109 



and insertion of its flagella, the distribution of the chloro- 

 phyl and location of the pigment spot, position of the 

 mouth if present, etc., etc. 



The record of this study may well consist in sketches, 

 and notes on the things observed. 



REPRODUCTION AMONG THE SIMPLER ORGANISMS. 



I. Cell division. — Increase of individuals is brought 

 about among the simpler organisms, as we have already seen, 

 by simple c-ell division, or fission. This process seems ordi- 

 narily to be initiated by the nucleus (which undergoes 

 changes to be described in a subsequent chapter) which 

 separates itself into two parts, about which the other cell 

 constituents become aggregated. The living substance of 

 the mother cell thus lives on in the two daughter cells. 



Cell division is an automatic, internal, spontaneous proc- 

 ess. It cannot be effected artificially; it cannot be com- 

 pelled or prcA^ented. If a cell be cut in two, the part of it 

 containing the nucleus and a part of the cytoplasm may 

 live, but the cytoplasm deprived of the nucleus dies, and a 

 wholly isolated nucleus dies also. The cell is the unit of 

 organic structure and function, and in it nucleus and 

 cytoplasm bear relations of mutual dependence. They 

 work together in both nutritiA^e and reproductive processes. 



Cell division results from growth, the increase of size dis- 

 turbing the nutritive relations between the living substance 

 and its food supply : for volume tends to increase faster than 

 surface ; the former (if the cell be spherical) as the cube, 

 the latter as the square of the diameter. Therefore, the 

 mass of protoplasm tends to increase more rapidly than 

 the surface through which it derives its nourishment and 

 expels its waste. This is a sufficient reason why cells are 

 small, and why, when a standard maximum size is reached, 

 fission sets in to keep them so. 



