Hi Debbie,
According to Brigid Hogan ('Manipulating the Mouse Embryo'), penetration of substrate becomes limiting after E13 and she suggests sagitally bisectioning of embryos after that time point. Plus she gives some protocols using paraformaldehyde or glutaraldehyde.  In our lab we use LacZ staining to examine developing limb in whole embryos up to E16 with 2% para, ears in E18-19 bisected heads.  Can you bisect your embryos, or are you going to perfuse the pregnant dam (Hogan has protocol for that, too)?  Maureen, Graduate Student at East Carolina University. 

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