198 



HUMAN BIOLOGY 



to a cell existing in a multicellular aggregate. It has more 

 individuality and is less bound down by the conventions 

 under which it lives. 



Fig. 3. Comparison of amcba and cell of intestine. 



An ameba (a) must adjust itself to its environment (e). A cell of intestine 

 (i) must react to its environment (e), neighboring cells (c and c') and fluids 

 of body (f). 



ALL CELLS ARE SAID TO DEVELOP FROM PRE-EXISTING 



CELLS 



Another postulate of the cell theory is that all cells 

 originate, or develop, only from preexisting cells. No 

 exception to this generalization has ever been observed. 

 But its unqualified acceptance involves the further assump- 

 tion that life commenced at some very remote epoch when 

 the primordial cell was first built up from presumably 

 lifeless components and that cells do not arise in this way 

 at the present time. As Wilson has clearly said life "is a 

 continuum, a never ending stream of protoplasm in the form 

 of cells, maintained by assimilation, growth and division. 

 The individual is but a passing eddy in the flow which 

 vanishes and leaves no trace, while the general stream of 

 life goes forward." Though this is what is always observed, 

 it is conceivable that exceptions, quite unsuspected, may in 



