Fig. 3. Alx1 protein expression in S. purpuratus (A-D) and L. variegatus (E-H) embryos. In both species, Alx1 protein is restricted to the nuclei of large micromere progeny (arrows). Expression is first detectable prior to PMC ingression and is evident throughout gastrulation. (A) 128-cell stage. (B) Mid-blastula stage; vegetal view. (C) Mesenchyme blastula. (D) An Alx1 MO-injected embryo that was allowed to develop until controls had reached the late gastrula stage. No nuclear staining is evident in cells of the vegetal plate (arrow). (E) Blastula. (F) Late gastrula. (G) Late gastrula stained with Alx1 and mAb 6a9, which recognizes a family of PMC-specific cell surface proteins. There is a one-to-one correspondence between 6a9- and Alx1-positive cells (arrow). (H) Overexpression of Xenopus C-cadherin. This embryo was fixed when sibling controls were late gastrulae. It has an animalized phenotype (Wikramanayake et al., 1998; Logan et al., 1999) and no Alx1 staining is detectable.