[toast]

I read it someplace. I didn't make it up.

14 days is when the cells begin to differentiate. It is a measured time. That time may be slightly different for each individual embryo but it is definite there is no brain function.

I read that there are some researchers that don't like to work with embryo's past this age because even they worry about these things.

[hopespringseternal]

I read it someplace. I didn't make it up.

You missed the point of my question. For one thing, you do not know the moment the embryo differentiates. For another, it is an arbitrary milestone. You have chosen it because it is convenient for you. For other people it might be the end of the first or second trimester or viability. But in the end, it is a fuzzy date tied to no particular event than can be readily measured.