Hi Michael
I have worked a lot with mouse embryos from 15 days and older. As they are 
tiny and very gelatinous, immersion fixation works very nicely, as long as 
you remove the embryo fast from it embryonic sack.
After removal, cut off the head, arms and legs for better penetration, (if 
you are not looking specific into these parts) and open the chest.

The embryo should be dropped into icecold fixative (preferably 4% 
paraformaldehyde in TBS buffer, but it depends on your use..) in a container 
kept on ice. Appropriate fixation time is from 1-4 hours.
The embryo should preferably be processed on a short program in the tissue 
processor.
If you have more questions, don't hesitate to ask.
Kind regards Andrea

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