You've used the phrase "by chance" and I don't think you realise how much of an implication that has.

There have been billlions and billions and conceptions in the history of the world, of which > 99.999% have successfully became to be the organism they're supposed to be, so saying a "just by chance a cell receives the exact correct amount of signals" seems so... unscientific to me. Especially if this process by chance happens a millions times just to form one embryo.

It just does not seem probable. A maths guy could probably work out a good estimate of the chances of all things happening, and I bet it'd be impossibly low.

We're talking about lifeless, non-intelligent bits of chemicals here.. And all it takes is "chance"? Come on man... The big bearded guy up in the sky seems more probable than this..