In a sea urchin, this begins with the dissociation of the nuclear envelope of the sperm, and the dispersal of chromatin to form the male pronucleus. The envelope of the male pronucleus is then fused with the envelope of the female pronucleus. Together they form a bridge-like structure and become one nucleus for the zygote. The initiation of DNA synthesis as well as a recondensing of the chromatin so that cleavage can occur follows this. The genome of the zygote is in charge of the regulation of cleavage, but the actual rate of cleavage is controlled by the ooplasm.
Cleavage is made up of two separate processes, karyokinesis and cytokinesis.
Karyokinesis is division of the nucleus, whereas cytokinesis is division of the cytoplasm. These two processes are coupled together so that the daughter nuclei will be equally distributed to each new cell. As cleavage continues, the cells develop into a morula, a round ball of cells with a fluid filled cavity called the blastocoele. It continues to increase in size as the cell divisions increase, but it becomes thinner and thinner as it develops into a blastula. The first thing the blastula must do in order to become a three-dimensional embryo, is that is must become two layered.