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(A) Summary of early development of the sea urchin embryo. Small micromeres or large micromeres and their corresponding descendants are indicated in red or blue, respectively. PMC; primary mesenchyme cell, SMC; secondary mesenchyme cells. (B) Transplantation of extra micromeres to the animal cap at the 16-cell stage induces the secondary gut. Host micromeres or transplanted micromeres and their descendants are indicated in red or blue, respectively.
