Chaetognatha, meaning bristle-jaws, known as arrow worms, is a phylum of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic, can attach to algae and rocks, they are found in all marine waters, from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped, they range in size from 2 to 120 millimetres. There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is large. Arrow worms are considered a type of protostome that do not belong to either Ecdysozoa or Lophotrochozoa. Chaetognaths are transparent or translucent dart-shaped animals covered by a cuticle; the body is divided into a distinct head and tail. There are between four and fourteen hooked, grasping spines on each side of their head, flanking a hollow vestibule containing the mouth; the spines are used in hunting, covered with a flexible hood arising from the neck region when the animal is swimming.
All chaetognaths are carnivorous. The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the fin rays of fish, with which they are not homologous, however: unlike those of vertebrates, these are composed of a thickened basement membrane extending from the epidermis. An additional caudal fin covers the post-anal tail. Two chaetognath species, Caecosagitta macrocephala and Eukrohnia fowleri, have bioluminescent organs on their fins. Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins with stabilization and steering; some species are known to use the neurotoxin tetrodotoxin to subdue prey. The body cavity is lined by peritoneum, therefore represents a true coelom, is divided into one compartment on each side of the trunk, additional compartments inside the head and tail, all separated by septa. Although they have a mouth with one or two rows of tiny teeth, compound eyes, a nervous system, they have no respiratory or circulatory systems.
The mouth opens into a muscular pharynx. From here, a straight intestine runs the length of the trunk to an anus just forward of the tail; the intestine is the primary site of digestion and includes a pair of diverticula near the anterior end. Materials are moved about the body cavity by cilia. Waste materials are excreted through the skin and anus; the nervous system is reasonably simple, consisting of a ganglionated nerve ring surrounding the pharynx. The dorsal ganglion is the largest, but nerves extend from all the ganglia along the length of the body. Chaetognaths have two compound eyes, each consisting of a number of pigment-cup ocelli fused together. In addition, there are a number of sensory bristles arranged in rows along the side of the body, where they perform a function similar to that of the lateral line in fish. An additional, band of sensory bristles lies over the head and neck; the arrow worm rhabdomeres are derived from microtubules 20 nm long and 50 nm wide, which in turn form conical bodies that contain granules and thread structures.
The cone body is derived from a cilium. All species are hermaphroditic, carrying sperm; each animal possesses a pair of testes within the tail, a pair of ovaries in the posterior region of the main body cavity. Immature sperm are released from the testes to mature inside the cavity of the tail, swim through a short duct to a seminal vesicle where they are packaged into a spermatophore. During mating, each individual places a spermatophore onto the neck of its partner after rupture of the seminal vesicle; the sperm escape from the spermatophore and swim along the midline of the animal until they reach a pair of small pores just in front of the tail. These pores connect to the oviducts, into which the developed eggs have passed from the ovaries, it is here that fertilisation takes place; the eggs are planktonic, or attached to algae, hatch into miniature versions of the adult, without a well-defined larval stage. Chaetognaths are traditionally classed as deuterostomes by embryologists. Lynn Margulis and K. V. Schwartz place chaetognaths in the deuterostomes in their Five Kingdom classification.
Molecular phylogenists, consider them to be protostomes. Thomas Cavalier-Smith places them in the protostomes in his Six Kingdom classification; the similarities between chaetognaths and nematodes mentioned above may support the protostome thesis—in fact, chaetognaths are sometimes regarded as a basal ecdysozoan or lophotrochozoan. Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny; this may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they might have retained deuterostome characters inherited from early bilaterian ancestors, thus chaetognaths may be a useful model for the ancestral bilaterian. Studies of arrow worms' nervous systems suggests. According to 2017 and 2019 papers, chaetognaths appear related to gnathiferans. Due to their soft bodies, chaetognaths fossilize poorly. So, several fossil chaetognath species have been described.
Chaetognaths appear to have originated in the Cambrian Period. Complete body fossils have been formally described from the Lower Cambrian Maotianshan shales of Yunnan, China and th
An earthworm is a tube-shaped, segmented worm found in the phylum Annelida. They have a world-wide distribution and are found living in soil, feeding on live and dead organic matter. An earthworm's digestive system runs through the length of its body, it conducts respiration through its skin. It has a double transport system composed of coelomic fluid that moves within the fluid-filled coelom and a simple, closed blood circulatory system, it has a peripheral nervous system. The central nervous system consists of two ganglia above the mouth, one on either side, connected to a nerve cord running back along its length to motor neurons and sensory cells in each segment. Large numbers of chemoreceptors are concentrated near its mouth. Circumferential and longitudinal muscles on the periphery of each segment enable the worm to move. Similar sets of muscles line the gut, their actions move the digesting food toward the worm's anus. Earthworms are hermaphrodites: each individual carries both male and female sex organs.
As invertebrates, they lack either an internal skeleton or exoskeleton, but maintain their structure with fluid-filled coelom chambers that function as a hydrostatic skeleton. "Earthworm" is the common name for the largest members of Oligochaeta. In classical systems, they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, though the internal male segments are anterior to the female. Theoretical cladistic studies have placed them, instead, in the suborder Lumbricina of the order Haplotaxida, but this may again soon change. Folk names for the earthworm include "dew-worm", "rainworm", "night crawler", "angleworm". Larger terrestrial earthworms are called megadriles, as opposed to the microdriles in the semiaquatic families Tubificidae and Enchytraeidae, among others; the megadriles are characterized by having a distinct clitellum and a vascular system with true capillaries. Depending on the species, an adult earthworm can be from 10 mm long and 1 mm wide to 3 m long and over 25 mm wide, but the typical Lumbricus terrestris grows to about 360 mm long.
The longest worm on confirmed records is Amynthas mekongianus that extends up to 3 m in the mud along the banks of the 4,350 km Mekong River in Southeast Asia. From front to back, the basic shape of the earthworm is a cylindrical tube, divided into a series of segments that compartmentalize the body. Furrows are externally visible on the body demarking the segments. Except for the mouth and anal segments, each segment carries bristle-like hairs called lateral setae used to anchor parts of the body during movement. Special ventral setae are used to anchor mating earthworms by their penetration into the bodies of their mates. Within a species, the number of segments found is consistent across specimens, individuals are born with the number of segments they will have throughout their lives; the first body segment features both the earthworm's mouth and, overhanging the mouth, a fleshy lobe called the prostomium, which seals the entrance when the worm is at rest, but is used to feel and chemically sense the worm's surroundings.
Some species of earthworm can use the prehensile prostomium to grab and drag items such as grasses and leaves into their burrow. An adult earthworm develops a belt-like glandular swelling, called the clitellum, which covers several segments toward the front part of the animal; this produces egg capsules. The posterior is most cylindrical like the rest of the body, but depending on the species, may be quadrangular, trapezoidal, or flattened; the last segment is called the periproct. The exterior of an individual segment is a thin cuticle over skin pigmented red to brown, which has specialized cells that secrete mucus over the cuticle to keep the body moist and ease movement through soil. Under the skin is a layer of nerve tissue, two layers of muscles—a thin outer layer of circular muscle, a much thicker inner layer of longitudinal muscle. Interior to the muscle layer is a fluid-filled chamber called a coelom that by its pressurization provides structure to the worm's boneless body; the segments are separated from each other by septa which are perforated transverse walls, allowing the coelomic fluid to pass between segments.
A pair of structures called. This tubule leads to the main body fluid filtering organ, the nephridium or metanephridium, which removes metabolic waste from the coelomic fluid and expels it through pores called nephridiopores on the worm's sides. At the center of a worm is the digestive tract, which runs straight through from mouth to anus without coiling, is flanked above and below by blood vessels and the ventral nerve cord, is surrounded in each segment by a pair of pallial blood vessels that connect the dorsal to th
Embryonic development embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal development describe stages. Embryonic development starts with the fertilization of the egg cell by a sperm cell. Once fertilized, the ovum is referred to a single diploid cell; the zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo. Although embryogenesis occurs in both animal and plant development, this article addresses the common features among different animals, with some emphasis on the embryonic development of vertebrates and mammals; the egg cell is asymmetric, having an "animal pole" and a "vegetal pole". It is covered with different layers; the first envelope – the one in contact with the membrane of the egg – is made of glycoproteins and is known as the vitelline membrane. Different taxa show different cellular and acellular envelopes englobing the vitelline membrane.
Fertilization is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation; the fertilized egg cell is known as the zygote. To prevent more than one sperm fertilizing the egg, fast block and slow block to polyspermy are used. Fast block, the membrane potential depolarizing and returning to normal, happens after an egg is fertilized by a single sperm. Slow block begins the first few seconds after fertilization and is when the release of calcium causes the cortical reaction, various enzymes releasing from cortical granules in the eggs plasma membrane, to expand and harden the outside membrane, preventing more sperm from entering. Cell division with no significant growth, producing a cluster of cells, the same size as the original zygote, is called cleavage.
At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula. The different cells derived from cleavage, up to the blastula stage, are called blastomeres. Depending on the amount of yolk in the egg, the cleavage can be holoblastic or meroblastic. Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via the placenta or milk, such as might be secreted from a marsupium. On the other hand, meroblastic cleavage occurs in animals; because cleavage is impeded in the vegetal pole, there is an uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote. 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: The end of cleavage is known as midblastula transition and coincides with the onset of zygotic transcription.
In amniotes, the cells of the morula are at first aggregated, but soon they become arranged into an outer or peripheral layer, the trophoblast, which does not contribute to the formation of the embryo proper, an inner cell mass, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, thus the morula is converted into a vesicle, called the blastodermic vesicle; the inner cell mass remains in contact, with the trophoblast at one pole of the ovum. After the 7th cleavage has produced 128 cells, the embryo is called a blastula; the blastula is a spherical layer of cells surrounding a fluid-filled or yolk-filled cavity Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass, distinct from the surrounding blastula. The blastocyst must not be confused with the blastula. In the mouse, primordial germ cells arise from a layer of cells in the inner cell mass of the blastocyst as a result of extensive genome-wide reprogramming.
Reprogramming involves global DNA demethylation facilitated by the DNA base excision repair pathway as well as chromatin reorganization, results in cellular totipotency. Before gastrulation, the cells of the trophoblast become differentiated into two strata: The outer stratum forms a syncytium, termed the syncytiotrophoblast, while the inner layer, the cytotrophoblast or "Layer of Langhans", consists of well-defined cells; as stated, the cells of the trophoblast do not contribute to the formation of the embryo proper. On the deep surface of the inner cell mass, a layer of flattened cells, called the endoderm, is differentiated and assumes the form of a small sac, called the yolk sac. Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the amniotic c
Deuterostomes comprise a superphylum of animals. It is a sister clade of Protostomia. Deuterostomia is a subtaxon of the Bilateria branch of the subkingdom Eumetazoa, within Animalia, are distinguished from protostomes by their deuterostomic embryonic development. Deuterostomes are known as enterocoelomates because their coelom develops through enterocoely. There are three major clades of deuterostomes: Chordata Echinodermata Hemichordata Previously, Deuterostomia included the phyla Brachiopoda, Bryozoa and Phoronida based on embryological characteristics. However, Superphylum Deuterostomia was redefined in 1995 based on DNA molecular sequence analyses when the lophophorates were removed from it and combined with other protostome animals to form superphylum Lophotrochozoa; the phylum Chaetognatha may belong here, but molecular studies have placed them in the protostomes more often. Extinct deuterostome groups may include the phylum Vetulicolia. Echinodermata and Hemichordata form the clade Ambulacraria.
In both deuterostomes and protostomes, a zygote first develops into a hollow ball of cells, called a blastula. In deuterostomes, the early divisions occur perpendicular to the polar axis; this is called radial cleavage, occurs in certain protostomes, such as the lophophorates. Most deuterostomes display indeterminate cleavage, in which the developmental fate of the cells in the developing embryo are not determined by the identity of the parent cell. Thus, if the first four cells are separated, each cell is capable of forming a complete small larva. In deuterostomes the mesoderm forms as evaginations of the developed gut that pinch off, forming the coelom; this is called enterocoely. Another feature present in both the Hemichordata and Chordata is pharyngotremy. A hollow nerve cord is found in all chordates, including tunicates; some hemichordates have a tubular nerve cord. In the early embryonic stage, it looks like the hollow nerve cord of chordates; because of the modified nervous system of echinoderms, it is not possible to discern much about their ancestors in this matter, but based on different facts it is quite possible that all the present deuterostomes evolved from a common ancestor that had pharyngeal gill slits, a hollow nerve cord and longitudinal muscles and a segmented body.
It could have resembled the small group of Cambrian urochordate deuterostomes named Vetulicolia. The defining characteristic of the deuterostome is the fact that the blastopore becomes the anus, whereas in protostomes the blastopore becomes the mouth; the deuterostome mouth develops at the opposite end of the embryo from the blastopore and a digestive tract develops in the middle, connecting the two. In many animals these early development stages evolved in ways that no longer reflect these original patterns. For instance, humans have formed a gut tube at the time of formation of the mouth and anus; the mouth forms first, during the fourth week of development, the anus forms four weeks temporarily forming a cloaca. The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes. Chordates are deuterostomes, it seems that the 555 million year old Kimberella was a member of the protostomes. That implies that the protostome and deuterostome lineages split some time before Kimberella appeared — at least 558 million years ago, hence well before the start of the Cambrian 541 million years ago, i.e. during the part of the Ediacaran Era.
The oldest discovered proposed deuterostome is Saccorhytus coronarius, which lived 540 million years ago. The researchers that made the discovery believe that the Saccorhytus is a common ancestor to all previously-known deuterostomes. Fossils of one major deuterostome group, the echinoderms, are quite common from the start of Series 2 of the Cambrian, 521 million years ago; the Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate. Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate. Another Chengjiang fossil, Haikouella lanceolata from the Chengjiang fauna, is interpreted as a chordate and a craniate, as it shows signs of a heart, gill filaments, a tail, a neural chord with a brain at the front end, eyes — although it had short tentacles round its mouth. Haikouichthys and Myllokunmingia from the Chengjiang fauna, are regarded as fish. Pikaia, discovered much earlier but from the Mid Cambrian Burgess Shale, is regarded as a primitive chordate.
On the other hand, fossils of early chordates are rare, as non-vertebrate chordates have no bone tissue or teeth, fossils of no Post-Cambrian non-vertebrate chordates are known aside from the Permian-aged Paleobranchiostoma, trace fossils of the Ordovician colonial tunicate Catellocaula, various Jurassic-
Symmetry in biology
Symmetry in biology is the balanced distribution of duplicate body parts or shapes within the body of an organism. In nature and biology, symmetry is always approximate. For example, plant leaves – while considered symmetrical – match up when folded in half. Symmetry creates a class of patterns in nature, where the near-repetition of the pattern element is by reflection or rotation; the body plans of most multicellular organisms exhibit some form of symmetry, whether radial, bilateral, or spherical. A small minority, notably among the sponges, exhibit no symmetry. Symmetry was once important in animal taxonomy. Radially symmetric organisms resemble a pie where several cutting planes produce identical pieces; such an organism exhibits no right sides. They have a front and a back. Symmetry has been important in the taxonomy of animals. Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, end.
Radial symmetry is suitable for sessile animals such as the sea anemone, floating animals such as jellyfish, slow moving organisms such as starfish. Animals in the phyla Cnidaria and Echinodermata are radially symmetric, although many sea anemones and some corals have bilateral symmetry defined by a single structure, the siphonoglyph. Many flowers are radially actinomorphic. Identical flower parts – petals and stamens – occur at regular intervals around the axis of the flower, the female part, with the carpel and stigma. Many viruses have radial symmetries, their coats being composed of a small number of protein molecules arranged in a regular pattern to form polyhedrons, spheres, or ovoids. Most are icosahedrons. Tetramerism is a variant of radial symmetry found in jellyfish, which have four canals in an otherwise radial body plan. Pentamerism means. Among animals, only the echinoderms such as sea stars, sea urchins, sea lilies are pentamerous as adults, with five arms arranged around the mouth.
Being bilaterian animals, they develop with mirror symmetry as larvae gain pentaradial symmetry later. Flowering plants show fivefold symmetry in various fruits; this is well seen in the arrangement of the five carpels in an apple cut transversely. Hexamerism is found in the corals and sea anemones which are divided into two groups based on their symmetry; the most common corals in the subclass Hexacorallia have a hexameric body plan. Octamerism is found in corals of the subclass Octocorallia; these have polyps with octameric radial symmetry. The octopus, has bilateral symmetry, despite its eight arms. Spherical symmetry occurs in an organism if it is able to be cut into two identical halves through any cut that runs through the organism's center. Organisms which show approximate spherical symmetry include the freshwater green alga Volvox. In bilateral symmetry, only one plane, called the sagittal plane, divides an organism into mirror image halves, thus there is approximate reflection symmetry. Internal organs are however not symmetric.
Animals that are bilaterally symmetric have mirror symmetry in the sagittal plane, which divides the body vertically into left and right halves, with one of each sense organ and limb group on either side. At least 99% of animals are bilaterally symmetric, including humans, where facial symmetry influences people's judgements of attractiveness; when an organism moves in one direction, it has a front or head end. This end encounters the environment before the rest of the body as the organism moves along, so sensory organs such as eyes tend to be clustered there, it is the site for a mouth as food is encountered. A distinct head, with sense organs connected to a central nervous system, therefore tends to develop. Given a direction of travel which creates a front/back difference, gravity which creates a dorsal/ventral difference and right are unavoidably distinguished, so a bilaterally symmetric body plan is widespread and found in most animal phyla. Bilateral symmetry permits streamlining to reduce drag, on a traditional view in zoology facilitates locomotion.
However, in the Cnidaria, different symmetries exist, bilateral symmetry is not aligned with the direction of locomotion, so another mechanism such as internal transport may be needed to explain the origin of bilateral symmetry in animals. The phylum Echinodermata, which includes starfish, sea urchins and sand dollars, is unique among animals in having bilateral symmetry at the larval stage, but pentamerism as adults. Bilateral symmetry is not broken. In experiments using the fruit fly, Drosophila, in contrast to other traits, right- or left-sidedness in eye size, or eye facet number, wing-folding behavior show a lack of response. Females of some species select for symmetry, presumed by biologists to be a ma
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the city of Perm. The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles and archosaurs; the world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors; the Permian ended with the Permian–Triassic extinction event, the largest mass extinction in Earth's history, in which nearly 96% of marine species and 70% of terrestrial species died out.
It would take well into the Triassic for life to recover from this catastrophe. Recovery from the Permian–Triassic extinction event was protracted; the term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Édouard de Verneuil; the region now lies in the Perm Krai of Russia. Official ICS 2017 subdivisions of the Permian System from most recent to most ancient rock layers are: Lopingian epoch Changhsingian Wuchiapingian Others: Waiitian Makabewan Ochoan Guadalupian epoch Capitanian stage Wordian stage Roadian stage Others: Kazanian or Maokovian Braxtonian stage Cisuralian epoch Kungurian stage Artinskian stage Sakmarian stage Asselian stage Others: Telfordian Mangapirian Sea levels in the Permian remained low, near-shore environments were reduced as all major landmasses collected into a single continent—Pangaea; this could have in part caused the widespread extinctions of marine species at the end of the period by reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean, the Paleo-Tethys Ocean, a large ocean that existed between Asia and Gondwana; the Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys Ocean to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic era. Large continental landmass interiors experience climates with extreme variations of heat and cold and monsoon conditions with seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea; such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores in a wetter environment. The first modern trees appeared in the Permian. Three general areas are noted for their extensive Permian deposits—the Ural Mountains and the southwest of North America, including the Texas red beds.
The Permian Basin in the U. S. states of Texas and New Mexico is so named because it has one of the thickest deposits of Permian rocks in the world. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an ice age. Glaciers receded around the mid-Permian period as the climate warmed, drying the continent's interiors. In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Permian marine deposits are rich in fossil mollusks and brachiopods. Fossilized shells of two kinds of invertebrates are used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist, one of the foraminiferans, ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct. Terrestrial life in the Permian included diverse plants, fungi and various types of tetrapods; the period saw a massive desert covering the interior of Pangaea.
The warm zone spread in the northern hemisphere. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals became marginal elements; the Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began; the swamp-loving
Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a multilayered structure known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells. In triploblastic organisms the gastrula is trilaminar; these three germ layers are known as the ectoderm and endoderm. In diploblastic organisms, such as Cnidaria and Ctenophora, the gastrula has only ectoderm and endoderm; the two layers are sometimes referred to as the hypoblast and epiblast. Gastrulation takes place after the formation of the blastula. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers; each layer gives rise to specific organs in the developing embryo. The ectoderm gives rise to epidermis, the nervous system, to the neural crest in vertebrates; the endoderm gives rise to the epithelium of the digestive system and respiratory system, organs associated with the digestive system, such as the liver and pancreas.
The mesoderm gives rise to many cell types such as muscle and connective tissue. In vertebrates, mesoderm derivatives include the notochord, the heart and blood vessels, the cartilage of the ribs and vertebrae, the dermis. Following gastrulation, cells in the body are either organized into sheets of connected cells, or as a mesh of isolated cells, such as mesenchyme; the molecular mechanism and timing of gastrulation is different in different organisms. However, some common features of gastrulation across triploblastic organisms include: A change in the topological structure of the embryo, from a connected surface, to a non-simply connected surface; the signaling pathways, which refers to the signals that indicate activation or inhibition of something else in the organism, are different depending on the organism as well. Lewis Wolpert, pioneering developmental biologist in the field, has been credited for noting that "It is not birth, marriage, or death, but gastrulation, the most important time in your life."The terms "gastrula" and "gastrulation" were coined by Ernst Haeckel, in his 1872 work "Biology of Calcareous Sponges".
Although gastrulation patterns exhibit enormous variation throughout the animal kingdom, they are unified by the five basic types of cell movements that occur during gastrulation: 1) invagination 2) involution 3) ingression 4) delamination 5) epiboly. Gastrulation is variable across the animal kingdom but has underlying similarities. Gastrulation has been studied in many animals, but some models have been used for longer than others. Furthermore, it is easier to study development in animals. Animals whose gastrulation is understood in the greatest detail include: Mollusc Sea urchin Frog Chicken The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore. Protostome derives from the Greek word protostoma meaning "first mouth" whereas Deuterostome's etymology is "second mouth" from the words second and mouth; the major distinctions between deuterostomes and protostomes are found in embryonic development: Mouth/anus In protostome development, the first opening in development, the blastopore, becomes the animal's mouth.
In deuterostome development, the blastopore becomes the animal's anus. Cleavage Protostomes have what is known as spiral cleavage, determinate, meaning that the fate of the cells is determined as they are formed. Deuterostomes have what is known as radial cleavage, indeterminate. Sea urchins Euechinoidea have been an important model system in developmental biology since the 19th century, their gastrulation is considered the archetype for invertebrate deuterostomes. Sea urchins exhibit stereotyped cleavage patterns and cell fates. Maternally deposited mRNAs establish the organizing center of the sea urchin embryo. Canonical Wnt and Delta-Notch signaling progressively segregate progressive mesoderm. In Euechinoids the first cells to internalize are the primary mesenchyme cells, which have a skeletogenic fate, which ingress during the blastula stage. Gastrulation – internalization of the prospective endoderm and non-skeletogenic mesoderm – begins shortly thereafter with invagination and other cell rearrangements the vegetal pole, which contribute 30% to the final archenteron length.
The gut's final length depends on cell rearrangements within the archenteron. Tailless amphibians are a classic model system for gastrulation; the sperm contributes one of the two mitotic asters needed to complete first cleavage. The sperm can enter anywhere in the animal half of the egg but its exact point of entry will break the egg's radial symmetry by organizing the cytoskeleton. Prior to first cleavage, the egg's cortex rotates relative to the internal cytoplasm by the coordinated action of microtubules, in a process known as cortical rotation; this displacement brings maternally loaded determinants of cell fate from the equatorial cytoplasm and vegetal cortex into contact, together these determinants set up the organizer. Thus, the area on the vegetal side opposite the sperm entry point will become the organizer. Hilde Mangold, working i