development - Why does cell differentiation start with the morula?

I suppose that cell differentation has to start by some external symmetry breaking. The first time there are two different types of cells is the morula with roughly 16 cells: there are some 12-15 cells forming the surface of the morula and some 1-4 cells in the interior of the morula. These are exposed to physically/chemically different local environments which might be the initial triggers of cell differentiation.

(source)

Questions:

1. 
   
   

   Is this kind of argument/picture correct?

   
   


2. 
   

   What are the significant differences in the environments of the outer and the inner blastomeres of the morula that trigger cell differentiation - and how do they do?