[gasbag]

There is a Radiolab episode about this too. They talk about all of the things that we don't know about that happen between days 14 and ~30 (working from memory), and also how the person who came up with the 14 day rule chose that number at least in part because, at the time, it was way out of reach and did not seem like it would ever be achievable.

http://www.radiolab.org/story/primitive-streak/