Practice question

Rotational holoblastic cleavage characterizes C. elegans and mammals, diverging clearly from spiral or discoidal modes seen in mollusks and birds. The zygote first divides unequally into larger anterior AB founder and smaller posterior P1 stem cell. Subsequent divisions occur at right angles, with each P lineage division producing a somatic founder and continuing P cell. Founders AB, MS, E, C, D generate distinct tissues, while P4 becomes germline via Z2 and Z3 primordial cells. This invariant lineage enables precise fate mapping, unlike yolk-rich eggs where cleavage is restricted and incomplete.