Re: what causes the process of differentiation to begin in an egg before....

  Hi Kendle.  Differentiation actually doesn't occur until after 
fertilization and cleavage.  By definition, a single, unfertilized egg 
can't go on and differentiate, since differentiation is basically fate 
differences between different cells and groups of cells.  A good place to 
look for an overview of fertilization and cleavage would be on the Oct.98 
Development Archive here on the Mad Scientist page.  Mike Klymkowsky, at 
Colorado University, answered a question by another person concerning this 
area with lots of useful web links.  
   As for differentiation, that begins to occur relatively early in the 
embryo, i assume depending on the species.  In C. Elegans, differences 
between cells and cell fate in the embryo are observed as early as the 4 
cell stage.  The actual mechanism by which differentiation is caused is 
still sort of up in the air.  It's assumed that it occurs through 
selective control of gene expression.  By that, I mean that while all 
cells have the same genes in them, in differentiated cells, only the set 
of genes that the particular type of cell needs are actually "on" in the 
cell.  So, muscle cells turn on and express muscle specific genes, while 
turning "off" all the other genes that are encoded in the genome.  
   The process by which differentiation begins in the very early embryo, 
and how cell fates are set up at that point, is still not very well 
understood at all.  Hope this helps a little, at least.  Good luck.
-Josh