Practice question

Sea urchin development features holoblastic cleavages generating relatively small coeloblastula. After successive divisions progressing through 16-cell, 32-cell and 60-cell stages, cells arrange as hollow epithelial sphere. By late blastula just before hatching, embryo contains approximately 120 cells arranged as single-layered epithelium surrounding blastocoel filled with blastocoelic fluid. Cells are ciliated and secrete hatching enzyme digesting fertilization envelope. Numbers vary slightly among species such as Lytechinus and Strongylocentrotus purpuratus, but classic morphological counts cite 120 cells comprising mesomeres, macromeres, small and large micromeres. This stage sets up primary mesenchyme ingression.