Unturned embryo with first appearence of somite pairs.
1-4 somites: The allantois extends further into the exocoelom and the maxiliary components of the 1st branchial arch become prominent. The preotic sulcus is visible in the 2-3 somite embryo. The cardiogenic plate begins to form and the foregut pocket is clearly visible.
5-7: The headfolds are particularly prominent and neural closure occurs in the region of the 4th and 5th somites, extending in both directions from this site. The optic placodes are first evident and become indented to form the optic pits. The heart rudiment develops rapidly. The allantois contacts the chorion at the end of this stage.
Absent: The 2nd branchial arch and >7 somites.
All scale bars in microns
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