Isolated micromeres from the 64-cell stage sea urchin embryo induce the animal hemisphere to form endoderm and mesoderm, resulting in a recognizable pluteus larva. This classic experiment demonstrates the inductive power of micromeres in embryonic patterning.

Correct Answer

(C) A recognizable pluteus larva​

Micromeres, located at the vegetal pole, contain maternal determinants and signaling factors like those involving β-catenin that specify nearby cells to form endoderm and mesenchyme. When combined with the animal hemisphere (fated for ectoderm), the micromeres reorganize the animal cells to produce a gut (archenteron) and skeletal elements, yielding a near-normal pluteus larva with cilia, arms, and digestive structures.​

Option Analysis

• (A) A ball of ectomesodermal cells: Incorrect. Animal hemisphere alone forms ectoderm only; micromeres shift fates toward mesoderm but drive full patterning beyond a simple ball.​

• (B) A ciliated ball of ectodermal cells: Incorrect. This describes animal hemisphere cultured alone (Driesch’s experiment), forming a hollow, ciliated ectodermal blastula without micromere induction.​

• (C) A recognizable pluteus larva: Correct, as micromeres induce archenteron invagination and spicule formation from animal cap cells.​

• (D) A ball of endodermal cells: Incorrect. Vegetal hemisphere alone might emphasize endoderm, but this combination includes animal ectoderm precursors re-specified for full larval body plan.​

Experiment Background

At the 64-cell stage, sea urchin embryos feature 40 animal mesomeres/macromeres (ectoderm/endoderm potential) and 24 micromeres (mesenchyme inducers). Horstadius’ recombination shows micromeres autonomously specify primary mesenchyme cells (PMCs) and signal adjacent tiers for vegetal fates, overriding animal bias. This reveals hierarchical induction in deuterostome development.​