Practice question

Micromeres are crucial organizers in sea urchin embryo. Fourth cleavage produces four small micromeres vegetally with autonomous skeletogenic potential and inductive ability via Delta-Notch signaling. If micromeres are removed microsurgically, remaining macromeres and mesomeres lack Delta signal needed for secondary mesenchyme and endoderm induction. Resulting embryos fail to gastrulate, remaining as hollow blastulae covered with long cilia called dauerblastulae, unable to form archenteron or calcareous spicules. This contrasts with normal larvae that form gut, skeleton, and pluteus arms, showing micromere requirement for both autonomous and conditional fates and patterning.