I would like to understand which mechanism triggers the first cell differentiation after *n* divisions.

I read [previous](https://biology.stackexchange.com/questions/3720/how-are-different-types-of-cells-created-from-zygote/8309#8309) [articles](https://biology.stackexchange.com/questions/3741/how-do-cells-know-what-type-to-differentiate-into) on SE and Wikipedia articles on [cellular differentiation](http://en.wikipedia.org/wiki/Cellular_differentiation) and [embryogenesis](http://en.wikipedia.org/wiki/Embryogenesis) but still fail to understand what exactly makes it so that starting from a given division cells suddenly start to be different.

Wikipedia claims that

> 
> In the first hours after fertilization, this cell divides into
>  identical cells. In humans, approximately four days after
>  fertilization and after several cycles of cell division, these cells
>  begin to specialize, forming a hollow sphere of cells, called a
>  blastocyst
> 
> 
> 

but do not explain ***why*** they begin to specialize.

I could imagine 

* that a cell has a "counter" on the number of divisions which triggers differentiation after a given amount of divisions
* or a chemical substance (either cell-borne or external) forces a change in the division

but why some cells would become "cell A" and some others "cell B"?

I am fairly sure that the biochemical mechanisms which regulate the life of a grown up mechanism can explain cellular differentiation (through hormones for instance) -- I am however interested by this specific moment, this *n-th* division where identical cells become differentiated.