Sea urchin micromeres produce signals that specify:
Micromeres provide inductive organizing cues to adjacent Veg2 macromere descendants specifying germ layers. They synthesize and secrete Wnt8 as short-range paracrine factor activating β-catenin signaling in neighbors and present Delta ligand on surface activating Notch receptor on Veg2 cells. Delta-Notch interaction triggers nuclear translocation of Suppressor of Hairless nic, turning on Gcm and secondary mesenchyme programs, while Wnt8 and early β-catenin drive endoderm specification via FoxA and GataE in Veg1/Veg2. These juxtacrine and paracrine signals together convert conditionally specified cells to endomesoderm. Without induction, animal cells remain ectodermal. Thus micromeres actively specify neighboring cells toward endomesodermal fates.
Ref: Sherwood & McClay, Development 1999, Delta-Notch induction of endomesoderm; Gilbert Chapter 8.