The process by which the zygote divides repeatedly to form a multicellular embryo is called cleavage. If the cleavage is complete it is called holoblastic and if it is partial it is called meroblastic. At the time of cleavage, the end of the egg containing the yolk is called the vegetative pole and the end containing the nucleus is called the animal pole.
Cleavage is of two types on the basis of swelling. These are-
(i) Indeterminate Cleavage: In embryonic development where every cell produced in the initial stage of cleavage has the capacity to form an embryo is called indeterminate cleavage.
(ii) Determinate cleavage: In embryonic development, the specific cells produced in the initial stage of cleavage possessing the ability to form an embryo is called determinate cleavage. Dr. Siddique Publications
Cleavage is of three types based on the cleavage plane. These are-
(i) Radial Cleavage: The cleavage process in which the cells of the zygote divide evenly and radially is called radial cleavage. Auricular cleavage occurs in animals of the order Arthropoda.
(ii) Bilateral cleavage: The process in which the zygote cleaves anteriorly up to the second cleavage and transversely along the midline from the third cleavage is called bilateral cleavage. This results in two rows of four cells each. Bilateral cleavage occurs in animals of the order Chordata.
(iii) Spiral Cleavage: After the anterior and transverse divisions of the zygote, during the third division, when the blastomeres of the animal pole change slightly with the blastomeres of the vegetal pole, it is called spiral cleavage. Spiral cleavage occurs in animals of the order Annelida and Mollusca.
Bilaterally symmetrical animals are divided into two groups based on cleavage and embryonic development.
(i) Protostomia: Cleavage in protostomia animals is spiral and definite. Their embryonic blastopore develops into a pore. Such as Annelida, Arthropoda and Mollusca.
(ii) Deuterostomia: Cleavage in Deuterostomia animals is spiral and indeterminate. Their embryonic blastopore develops into an anus. E.g. Echinodermata and Chordata.