In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total or entire cleavage) or meroblastic (partial cleavage). The different cells derived from cleavage (up to the blastula stage) are called blastomeres. Embryology is the branch of developmental biology that studies embryos and their development. ... Cells in culture, stained for keratin (red) and DNA (green). ... Embryos (and one tadpole) of the wrinkled frog (Rana rugosa). ... This article or section does not cite its references or sources. ... The cell cycle, or cell-division cycle (CDC), is the series of events in a eukaryotic cell between one cell division and the next. ... The egg yolk is the yellow inside an egg. ... A blastomere is the structure which results from the divisions of a fertilised egg during embryonic development . ...

See also: embryogenesis. Embryogenesis is the process by which the embryo is formed and develops. ...

Holoblastic

In the absence of a large concentration of yolk, four major cleavage types can be observed in isolecithal cells - bilateral holoblastic, radial holoblastic, rotational holoblastic, and spiral holoblastic, cleavage. These holoblastic cleavage planes pass all the way through isolecithal zygotes during the process of cytokinesis. Coeloblastula is the next stage of development for eggs that undergo these radial cleavaging. In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms.

Bilateral

The first cleavage results in bisection of the zygote into left and right halves. The following cleavage planes are centered on this axis and result in the two halves being mirror images of one another. In bilateral holoblastic cleavage, the divisions of the blastomeres are complete and separate; compared with bilateral meroblastic cleavage, in which the blastomeres stay partially connected.

Radial

Radial cleavage is characteristic of the deuterostomes, which include some vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte. Phyla Chaetognatha Echinodermata Hemichordata Chordata Deuterostomes (from the Greek: second the mouth) are one of the two superphyla of animals in the taxonomic branch bilateria, the other being the protostomes. ... Classes and Clades See below Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. ... Classes Asteroidea Blastoidea (extinct) Concentricycloidea Crinoidea Echinoidea Holothuroidea Ophiuroidea Echinoderms (Phylum Echinodermata, from the Greek for spiny skin) are a phylum of marine animals found at all depths. ... An oocyte or ovocyte is a female gametocyte or germ cell involved in reproduction. ...

Rotational

Mammals display rotational cleavage, and an isolecithal distribution of yolk (sparsely and evenly distributed). Because the cells have only a small amount of yolk, they require immediate implantation onto the uterine wall in order to receive nutrients. Orders Multituberculata (extinct) Palaeoryctoides (extinct) Triconodonta (extinct) Subclass Australosphenida Ausktribosphenida Monotremata Subclass Eutheria (excludes extinct ancestors) Afrosoricida Anagaloidea (extinct) Arctostylopida (extinct) Artiodactyla Carnivora Cetacea Chiroptera Cimolesta (extinct) Cingulata Creodonta (extinct) Condylarthra (extinct) Dermoptera Desmostylia (extinct) Dinocerata (extinct) Embrithopoda (extinct) Hyracoidea Insectivora Lagomorpha Leptictida (extinct) Litopterna (extinct) Macroscelidea Mesonychia (extinct) Notoungulata... Isolecithal (Greek iso = equal, lekithos = yolk) is the even distribution of yolk in the cytoplasm of ovums of mammals and other invertebrates notably Petromyzontidae, Acipenseridae, Amiidae, and Lepisosteidae. ...

Rotational cleavage involves a normal first division along the meridional axis, giving rise to two daughter cells. The way in which this cleavage differs is that one of the daughter cells divides meridionally, whilst the other divides equatorially.

Spiral

In spiral cleavage, the cleavage planes are oriented obliquely to the polar axis of the oocyte. At the third cleavage the halves are oblique to the polar axis and typically produce an upper quartet of smaller cells that come to be set between the furrows of the lower quartet. All groups showing spiral cleavage are protostomia, such as annelids and mollusks. This article needs cleanup. ... Phyla Mollusca Arthropoda Annelida Protostomes (from the Greek: first the mouth) are a superphylum of animals in the taxonomic group bilateria, and include animals such as arthropods, mollusks, and nematodes. ... Classes and subclasses Class Polychaeta      (paraphyletic?) Class Clitellata    Oligochaeta -      (Earthworms, etc. ... Classes Caudofoveata Aplacophora Polyplacophora Monoplacophora Bivalvia Scaphopoda Gastropoda Cephalopoda † Rostroconchia The mollusks or molluscs are the large and diverse phylum Mollusca, which includes a variety of familiar creatures well-known for their decorative shells or as seafood. ...

Meroblastic

In the presence of a large amount of yolk in the fertilized egg cell, the cell can undergo partial, or meroblastic, cleavage. Two major types of meroblastic cleavage are discoidal and superficial.

Discoidal

In discoidal cleavage, the cleavage furrows do not penetrate the yolk. The embryo forms a disc of cells, called a blastodisc, on top of the yolk. Discoidal cleavage is commonly found in birds, reptiles, and fish which have telolecithal egg cells (egg cells with the yolk concentrated at one end). For other meanings of bird, see bird (disambiguation). ... Orders  Crocodilia - Crocodilians scary crocodiles. ... The Guppy (Poecilia reticulata) is one of the most popular freshwater aquarium fish species in the world. ...

Superficial

In superficial cleavage, mitosis occurs but not cytokinesis, resulting in a polynuclear cell. With the yolk positioned in the center of the egg cell, the nuclei migrate to the periphery of the egg, and the plasma membrane grows inward, partitioning the nuclei into individual cells. Superficial cleavage occurs in arthropods which have centrolecithal egg cells (egg cells with the yolk located in the center of the cell). A cell in early anaphase Mitosis is the process by which a cell separates its duplicated genome into two identical halves. ... A cell that has almost completed cytokinesis. ... Subphyla and Classes Subphylum Trilobitomorpha Trilobita - Trilobites (extinct) Subphylum Chelicerata Arachnida - Spiders, Scorpions, etc. ... Centrolecithal (Greek kentron = center of a circle, lekithos = yolk) is the placement of the yolk in the centre of the cytoplasm of ovums. ...

Classes Ascidiacea Thaliacea Appendicularia Sorberacea Urochordata (sometimes known as tunicata and commonly called urochordates, tunicates, sea squirts or cunji) is the subphylum of saclike filter feeders with input and output siphons. ... Subclasses and Orders Order Temnospondyli- extinct Subclass Lepospondyli- extinct Subclass Lissamphibia   Anura   Caudata   Gymnophiona Amphibians (class Amphibia) are a taxon of animals that include all tetrapods (four-legged vertebrates) that do not have amniotic eggs. ... Subclasses Euechinoidea Superorder Atelostomata Order Cassiduloida Order Spatangoida (heart urchins) Superorder Diadematacea Order Diadematoida Order Echinothurioida Order Pedinoida Superorder Echinacea Order Arbacioida Order Echinoida Order Phymosomatoida Order Salenioida Order Temnopleuroida Superorder Gnathostomata Order Clypeasteroida (sand dollars) Order Holectypoida Perischoechinoidea Order Cidaroida (pencil urchins) Sea urchins are spiny sea creatures of... Families Asymmetronidae Branchiostomidae The lancelets (subphylum Cephalochordata, and traditionally known as the amphioxus) are a group of primitive chordates. ... Orders Multituberculata (extinct) Palaeoryctoides (extinct) Triconodonta (extinct) Subclass Australosphenida Ausktribosphenida Monotremata Subclass Eutheria (excludes extinct ancestors) Afrosoricida Anagaloidea (extinct) Arctostylopida (extinct) Artiodactyla Carnivora Cetacea Chiroptera Cimolesta (extinct) Cingulata Creodonta (extinct) Condylarthra (extinct) Dermoptera Desmostylia (extinct) Dinocerata (extinct) Embrithopoda (extinct) Hyracoidea Insectivora Lagomorpha Leptictida (extinct) Litopterna (extinct) Macroscelidea Mesonychia (extinct) Notoungulata... Classes and subclasses Class Polychaeta      (paraphyletic?) Class Clitellata    Oligochaeta -      (Earthworms, etc. ... Classes Caudofoveata Aplacophora Polyplacophora Monoplacophora Bivalvia Scaphopoda Gastropoda Cephalopoda † Rostroconchia The mollusks or molluscs are the large and diverse phylum Mollusca, which includes a variety of familiar creatures well-known for their decorative shells or as seafood. ... The Guppy (Poecilia reticulata) is one of the most popular freshwater aquarium fish species in the world. ... Orders Many - see section below. ... Orders Procolophonia (extinct) Testudines Araeoscelidia (extinct) Avicephala (extinct) Younginiformes (extinct) Sauropterygia Ichthyosauria (extinct) Placodontia (extinct) Nothosauria (extinct) Plesiosauria (extinct) Sphenodontia Squamata Prolacertiformes (extinct) Archosauria Crurotarsi Order Aetosauria Order Phytosauria Order Rauisuchia Order Crocodilia Ornithodira Pterosauria (extinct) Marasuchus (extinct) Dinosauria (extinct) Order Saurischia Order Ornithischia Reptiles are tetrapods and amniotes, animals... Classes & Orders See taxonomy Insects are invertebrate animals of the Class Insecta, the largest and (on land) most widely-distributed taxon within the phylum Arthropoda. ...

References

• Wilt, F. & Hake, S. (2004). Principles of Developmental Biology.
• Scott F. Gilbert (2003). Developmental Biology.

See also: embryogenesis. Embryogenesis is the process by which the embryo is formed and develops. ...