Practice question

Sea urchin eggs are isolecithal with sparse evenly dispersed yolk, permitting complete holoblastic division of the entire zygote. Early cleavages follow a radial pattern where blastomeres remain aligned directly above one another, preserving symmetry around the animal-vegetal axis. First and second cleavages are meridional, third equatorial, with alternating orientations thereafter. Such radial holoblastic cleavage characterizes deuterostomes and supports highly regulative development. It contrasts sharply with spiral cleavage of protostomes where spindles tilt obliquely, or meroblastic and superficial modes seen in yolk-rich eggs.