Inhibition of FGFR activity blocks cell migration from the epiblast through the primitive streak to the mesoderm. A-B: Combined brightfield and fluorescence images of control (A) or SU5402 treated (B) embryos. Embryos were treated for two hours with DMSO carrier or SU5402, and then electroporated with a GFP expression construct. A'-A": indicates transverse sections through the control embryo in A; B'-B": represents transverse sections through the embryo shown in B. C: Quantification of GFP-expressing cell location at 5 hours after electroporation. EPIBLAST includes preingression epiblast and lateral epiblast extending to the area pellucida-opaca border; STREAK includes the primitive streak; MESODERM includes the medial and lateral mesoderm regions to the area pellucida-opaca border (see Figure 1A for depiction of domains). Migration of cells to the mesoderm is essentially abolished in SU5402 treated embryos. Abbreviations: PS, primitive streak; EP, epiblast; MS, mesoderm; EN, endoderm.