Additional Supporting Information may be found in the online version of this article.

DVDY_23711_sm_SuppFigS1.tif1835KSupporting Information Fig. S1. Germ layers can be identified by cell morphology in mT/mG embryos and embryo sections. A–C: Images of three different optical Z planes within an early-streak stage mT/mG embryo: A shows morphology of the epiblast layer, B shows morphology of the mesoderm layer, C shows morphology of the endoderm layer. Red is immuno-staining for T (Brachyury) to confirm mesoderm. Some cells within the nascent mesoderm layer demonstrate a round, bulbous appearance (arrowheads), which is the result of apical constriction as cells ingress at the streak.
DVDY_23711_sm_SuppMovieS1.mov3375KSupporting Information Movie S1. Time-lapse movie of a live e6.5 mT/mG embryo. This embryo is prestreak at the beginning of the movie, but is early streak (i.e., has mesoderm) by the movie's end.
DVDY_23711_sm_SuppMovieS2.mov2641KSupporting Information Movie S2. Time-lapse movie of a live no-bud stage mT/mG embryo. Primitive streak is seen en face at the level of the epiblast. Large, bulbous cells can be seen passing through the plane of view at varied times and locations as they ingress through the streak.

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