This type of cleavage is called as spiral cleavage. In protostomes, these early divisions occur at right angles parallel to the animal-vegetal axis of the zygote. During later developmental stages blastopore may develop either into mouth or anus. When the blastopore is not fully formed it is called virtual blastopore. Occasionally the invagination during gastrulation may be incomplete and the blastopore is not fully formed which results in the formation of a primitive streak as seen in the embryo of bird or mammal.

And the cavity between the two layers is Blastocoel. The outer layer of gastrula is the ectoderm and the inner layer is called as endoderm. The open cavity of gastrula lined by endoderm is called Archenteron or primitive gut. Invagination of endoderm and mesoderm cells at one end of the blastula is called gastrulation and this result in the formation of Gastrula. The central cavity of blastula is called as blastocoels. As the cleavage proceeds, the number of blastomeres increases and cells of blastula are arranged into a hollow ball like structure. Cleavage ends with the formation of Blastula. The different cells derived from the cleavage are called as blastomeres. During the early embryonic stages of animal development, zygote undergoes rapid cell divisions called as Cleavage to produce a compact mass of cells called Morula. The major difference between deuterostomes and protostomes are found in early embryonic development. Protostomes and deuterostomes along with few other smaller phyla constitute the grade bilateria.

Asterias blastopore