At around 3 weeks old the changes continue... Over the last week one of the most evident changes has been the differences in sizes within the population. Previously they had all grown at about the same rate and appeared roughly the same morphologically the same. Now some are clearly larger (around 30%) than most of the population while a few don't appear to be growing in size.
The larger ones seem to have increased complexity within the tissues - more definition of the internal organs including the size of the digestive tract and the musculature of the mouth. More interestingly they also appear to be developing visible nerves and bone plates.
A few have begun to differentiate the tissue around the mouth giving enough movement to make tearing off bits of plant material more efficient. Many are developing small whitish lumps behind the rear vent of the digestive tract - I am uncertain whether these will develop into legs...
They all seem to have increasing ability to direct the eye position but none seem to have iris control yet. They are much more agile in movement than last week. Social behavior has increased for most of them (moving in groups, resting in groups, periodic contact/bumping) but there are a few that seem to prefer to be alone.