Figure 3:

Cell movements and gene expression during chick and mouse gastrulation. (a) Polonaise movement of epiblast cells associated with primitive streak elongation in the developing chick embryo, viewed from above the dorsal side. (b) Mouse gastrulation at E6.5–E8.5 from primitive streak formation, elongation, and tail bud stage, viewed from a lateral side (c) Transverse sections of E7.5 mouse embryos. Snail1 is expressed in a small population of cells in the epiblast at the primitive streak (red bracket), in the nascent mesoderm (white arrow), and in the migratory mesoderm (yellow arrowheads). Twist is not expressed at the primitive streak or in the nascent mesoderm but is turned on later in the migratory mesoderm. Images courtesy of Nitya Ramkumar.