The cleavage initiation site establishes the posterior pole of the hydrozoan embryo.

When the first cleavage of the hydrozoan egg is reversibly suppressed, two cleavage furrows frequently form simultaneously at the time of the second cleavage. If these two cleavage initiation sites are far enough apart, each one specifies a site of gastrulation, and the embryo that forms develops into a two tailed planula larva. When two tailed planulae are induced to metamorphose, they form a polyp with two stalks and hydranths.