{"id":1867,"date":"2017-01-30T23:38:55","date_gmt":"2017-01-30T23:38:55","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=1867"},"modified":"2017-07-05T17:10:14","modified_gmt":"2017-07-05T17:10:14","slug":"animal-reproduction","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/chapter\/animal-reproduction\/","title":{"raw":"Animal Reproduction","rendered":"Animal Reproduction"},"content":{"raw":"<h2>Discuss methods and features of animal reproduction<\/h2>\r\nMost animals are diploid organisms, meaning that their body (somatic) cells are diploid and haploid reproductive (gamete) cells are produced through meiosis. Some exceptions exist: for example, in bees, wasps, and ants, the male is haploid because it develops from unfertilized eggs. Most animals undergo sexual reproduction. This fact distinguishes animals from fungi, protists, and bacteria, where asexual reproduction is common or exclusive. However, a few groups, such as cnidarians, flatworm, and roundworms, undergo asexual reproduction, although nearly all of those animals also have a sexual phase to their life cycle.\r\n<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\n<ul>\r\n \t<li>Explain the processes of animal reproduction and embryonic development<\/li>\r\n \t<li>Compare and contrast the embryonic development of protostomes and deuterostomes<\/li>\r\n \t<li>Describe the roles that Hox genes play in development<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h2>Animal Reproduction and Development<\/h2>\r\nDuring sexual reproduction, the haploid gametes of the male and female individuals of a species combine in a process called fertilization. Typically, the small, motile male sperm fertilizes the much larger, sessile female egg. This process produces a diploid fertilized egg called a zygote.\r\n\r\nSome animal species\u2014including sea stars and sea anemones, as well as some insects, reptiles, and fish\u2014are capable of asexual reproduction. The most common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation, where part of a parent individual can separate and grow into a new individual. In contrast, a form of asexual reproduction found in certain insects and vertebrates is called parthenogenesis (or \u201cvirgin beginning\u201d), where unfertilized eggs can develop into new male offspring. This type of parthenogenesis is called haplodiploidy. These types of asexual reproduction produce genetically identical offspring, which is disadvantageous from the perspective of evolutionary adaptability because of the potential buildup of deleterious mutations. However, for animals that are limited in their capacity to attract mates, asexual reproduction can ensure genetic propagation.\r\n\r\n[caption id=\"attachment_1675\" align=\"alignright\" width=\"399\"]<img class=\" wp-image-1675\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25210444\/Figure_27_01_02-1024x862.jpg\" alt=\"Illustration A shows the egg, nymph and adult stages of a grasshopper. The nymph stages are similar in appearance to the adult stage, but smaller. Illustration B shows the egg, larvae, pupa and adult stages of a butterfly. The pupa is a cocoon the butterfly makes when transforming from the larval to adult stages. The winged adult butterfly looks nothing like the caterpillar larva.\" width=\"399\" height=\"336\" \/> Figure\u00a01. (a) The grasshopper undergoes incomplete metamorphosis. (b) The butterfly undergoes complete metamorphosis. (credit: S.E. Snodgrass, USDA)[\/caption]\r\n\r\nAfter fertilization, a series of developmental stages occur during which primary germ layers are established and reorganize to form an embryo. During this process, animal tissues begin to specialize and organize into organs and organ systems, determining their future morphology and physiology. Some animals, such as grasshoppers, undergo incomplete metamorphosis, in which the young resemble the adult. Other animals, such as some insects, undergo complete metamorphosis where individuals enter one or more larval stages that may in differ in structure and function from the adult (Figure\u00a01). For the latter, the young and the adult may have different diets, limiting competition for food between them. Regardless of whether a species undergoes complete or incomplete metamorphosis, the series of developmental stages of the embryo remains largely the same for most members of the animal kingdom.\r\n\r\nThe process of animal development begins with the <strong>cleavage<\/strong>, or series of mitotic cell divisions, of the zygote (Figure\u00a02). Three cell divisions transform the single-celled zygote into an eight-celled structure. After further cell division and rearrangement of existing cells, a 6\u201332-celled hollow structure called a <strong>blastula<\/strong> is formed. Next, the blastula undergoes further cell division and cellular rearrangement during a process called gastrulation. This leads to the formation of the next developmental stage, the <strong>gastrula<\/strong>, in which the future digestive cavity is formed. Different cell layers (called <strong>germ layers<\/strong>) are formed during gastrulation. These germ layers are programmed to develop into certain tissue types, organs, and organ systems during a process called <strong>organogenesis<\/strong>.\r\n\r\n[caption id=\"attachment_1676\" align=\"aligncenter\" width=\"800\"]<img class=\"size-full wp-image-1676\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25210530\/Figure_27_01_03.jpg\" alt=\"The left part of the illustration shows a single-celled zygote. The initial cleavage, or cell division, results in a ball of cells, called the eight-cell stage. The cells do not grow during cleavage, so the eight-cell stage ball is about the same diameter as the zygote. Further cleavage results in a hollow ball of cells called a blastula. Upon gastrulation, part of the ball of cells invaginates, forming a cavity called a blastopore.\" width=\"800\" height=\"268\" \/> Figure\u00a02. During embryonic development, the zygote undergoes a series of mitotic cell divisions, or cleavages, to form an eight-cell stage, then a hollow blastula. During a process called gastrulation, the blastula folds inward to form a cavity in the gastrula.[\/caption]\r\n\r\n<div class=\"textbox\">\r\n\r\nWatch the following video to see how human embryonic development (after the blastula and gastrula stages of development) reflects evolution:\r\n\r\n<script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1593769\/plugins\/11085.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script>\r\n<iframe id=\"myytplayer\" src=\"https:\/\/www.youtube.com\/embed\/uAZmLYWEPGk?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe>\r\n\r\n<\/div>\r\n<h2>Embryological Development<\/h2>\r\nMost animal species undergo a separation of tissues into germ layers during embryonic development. Recall that these germ layers are formed during gastrulation, and that they are predetermined to develop into the animal\u2019s specialized tissues and organs. Animals develop either two or three embryonic germs layers (Figure\u00a03). The animals that display radial symmetry develop two germ layers, an inner layer (endoderm) and an outer layer (ectoderm). These animals are called <b>diploblasts<\/b>. Diploblasts have a non-living layer between the endoderm and ectoderm. More complex animals (those with bilateral symmetry) develop three tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). Animals with three tissue layers are called <b>triploblasts<\/b>.\r\n\r\n[caption id=\"attachment_1692\" align=\"aligncenter\" width=\"469\"]<img class=\"size-full wp-image-1692\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25213129\/Figure_27_02_04.png\" alt=\"The left illustration shows the two embryonic germ layers of a diploblast. The inner layer is the endoderm, and the outer layer is the ectoderm. Sandwiched between the endoderm and the ectoderm is a non-living layer. Right illustration shows the three embryonic germ layers of a triploblast. Like the diploblast, the triploblast has an inner endoderm and an outer ectoderm. Sandwiched between these two layers is a living mesoderm.\" width=\"469\" height=\"269\" \/> Figure\u00a03.\u00a0During embryogenesis, diploblasts develop two embryonic germ layers: an ectoderm and an endoderm. Triploblasts develop a third layer\u2014the mesoderm\u2014between the endoderm and ectoderm.[\/caption]\r\n\r\n<div class=\"textbox exercises\">\r\n<h3>Practice Question<\/h3>\r\nWhich of the following statements about diploblasts and triploblasts is false?\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>Animals that display radial symmetry are diploblasts.<\/li>\r\n \t<li>Animals that display bilateral symmetry are triploblasts.<\/li>\r\n \t<li>The endoderm gives rise to the lining of the digestive tract and the respiratory tract.<\/li>\r\n \t<li>The mesoderm gives rise to the central nervous system.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"815922\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"815922\"]Statement d is false.[\/hidden-answer]\r\n\r\n<\/div>\r\nEach of the three germ layers is programmed to give rise to particular body tissues and organs. The endoderm gives rise to the lining of the digestive tract (including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures. The ectoderm develops into the outer epithelial covering of the body surface, the central nervous system, and a few other structures. The mesoderm is the third germ layer; it forms between the endoderm and ectoderm in triploblasts. This germ layer gives rise to all muscle tissues (including the cardiac tissues and muscles of the intestines), connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen.\r\n<h3>Presence or Absence of a Coelom<\/h3>\r\nFurther subdivision of animals with three germ layers (triploblasts) results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a <b>coelom<\/b>, and those that do not. This epithelial cell-lined coelomic cavity represents a space, usually filled with fluid, which lies between the visceral organs and the body wall. It houses many organs such as the digestive system, kidneys, reproductive organs, and heart, and contains the circulatory system. In some animals, such as mammals, the part of the coelom called the pleural cavity provides space for the lungs to expand during breathing. The evolution of the coelom is associated with many functional advantages. Primarily, the coelom provides cushioning and shock absorption for the major organ systems. Organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility.\r\n\r\nTriploblasts that do not develop a coelom are called <b>acoelomates<\/b>, and their mesoderm region is completely filled with tissue, although they do still have a gut cavity. Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called <b>eucoelomates<\/b> (or coelomates) (Figure\u00a04). A true coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom derived partly from mesoderm and partly from endoderm, which is found between the two layers. Although still functional, these are considered false coeloms, and those animals are called <b>pseudocoelomates<\/b>. The phylum Nematoda (roundworms) is an example of a pseudocoelomate. True coelomates can be further characterized based on certain features of their early embryological development.\r\n\r\n[caption id=\"attachment_3573\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-3573\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/23214523\/Figure_27_02_05abc-1024x680.jpg\" alt=\"Part a shows the body plan of acoelomates, including flatworms. Acoelomates have a central digestive cavity. Outside this digestive cavity are three tissue layers: an inner endoderm, a central mesoderm, and an outer ectoderm. The photo shows a swimming flatworm, which has the appearance of a frilly black and pink ribbon. Part b shows the body plan of eucoelomates, which include annelids, mollusks, arthropods, echinoderms, and chordates. Eucoelomates have the same tissue layers as acoelomates, but a cavity called a coelom exists within the mesoderm. The coelom is divided into two symmetrical parts that are separated by two spokes of mesoderm. The photo shows a swimming annelid known as a bloodworm. The bloodworm has a tubular body that tapers at each end. Numerous appendages radiate from either side. Part c shows the body plan of pseudocoelomates, which include roundworms. Like the acoelomates and eucoelomates, the pseudocoelomates have an endoderm, a mesoderm, and an ectoderm. However, in pseudocoelomates, a pseudocoelum separates the endoderm from the mesoderm. The photo shows a roundworm, or nematode, which has a tubular body.\" width=\"1024\" height=\"680\" \/> Figure\u00a04.\u00a0Triploblasts may be (a) acoelomates, (b) eucoelomates, or (c) pseudocoelomates. Acoelomates have no body cavity. Eucoelomates have a body cavity within the mesoderm, called a coelom, which is lined with mesoderm. Pseudocoelomates also have a body cavity, but it is sandwiched between the endoderm and mesoderm. (credit a: modification of work by Jan Derk; credit b: modification of work by NOAA; credit c: modification of work by USDA, ARS)[\/caption]\r\n<h3>Embryonic Development of the Mouth<\/h3>\r\n[caption id=\"attachment_1695\" align=\"alignright\" width=\"350\"]<img class=\" wp-image-1695\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25213455\/Figure_27_02_06.jpg\" alt=\"The illustration compares the development of protostomes and deuterostomes. In both protostomes and deuterostomes, the gastrula, which resembles a hollow ball of cells, contains an indentation called a blastopore. In protostomes, two circular layers of mesoderm form inside the gastrula, containing the coelom cavity. As the protostome develops, the mesoderm grows and fuses with the gastrula cell layer. The blastopore becomes the mouth, and a second opening forms opposite the mouth, which becomes the anus. In deuterostomes, two groups of gastrula cells in the blastopore grow inward to form the mesoderm. As the deuterostome develops, the mesoderm pinches off and fuses, forming a second body cavity. The body plan of the deuterostome at this stage looks very similar to that of the protostome, but the blastopore becomes the anus, and the second opening becomes the mouth.\" width=\"350\" height=\"380\" \/> Figure\u00a05.\u00a0Eucoelomates can be divided into two groups based on their early embryonic development. In protostomes, part of the mesoderm separates to form the coelom in a process called schizocoely. In deuterostomes, the mesoderm pinches off to form the coelom in a process called enterocoely. It was long believed that the blastopore developed into the mouth in protostomes and into the anus in deuterostomes, but recent evidence challenges this belief.[\/caption]\r\n\r\nBilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in their early embryonic development. <b>Protostomes<\/b> include arthropods, mollusks, and annelids. <b>Deuterostomes<\/b> include more complex animals such as chordates but also some simple animals such as echinoderms. These two groups are separated based on which opening of the digestive cavity develops first: mouth or anus. The word protostome comes from the Greek word meaning \u201cmouth first,\u201d and deuterostome originates from the word meaning \u201cmouth second\u201d (in this case, the anus develops first). The mouth or anus develops from a structure called the blastopore (Figure\u00a05).\r\n\r\nThe <b>blastopore<\/b> is the indentation formed during the initial stages of gastrulation. In later stages, a second opening forms, and these two openings will eventually give rise to the mouth and anus (Figure\u00a05). It has long been believed that the blastopore develops into the mouth of protostomes, with the second opening developing into the anus; the opposite is true for deuterostomes. Recent evidence has challenged this view of the development of the blastopore of protostomes, however, and the theory remains under debate.\r\n\r\nAnother distinction between protostomes and deuterostomes is the method of coelom formation, beginning from the gastrula stage. The coelom of most protostomes is formed through a process called <b>schizocoely<\/b>, meaning that during development, a solid mass of the mesoderm splits apart and forms the hollow opening of the coelom. Deuterostomes differ in that their coelom forms through a process called <b>enterocoely<\/b>. Here, the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse to form the mesoderm, which then gives rise to the coelom.\r\n\r\nThe earliest distinction between protostomes and deuterostomes is the type of cleavage undergone by the zygote. Protostomes undergo <b>spiral cleavage<\/b>, meaning that the cells of one pole of the embryo are rotated, and thus misaligned, with respect to the cells of the opposite pole. This is due to the oblique angle of the cleavage. Deuterostomes undergo <b>radial cleavage<\/b>, where the cleavage axes are either parallel or perpendicular to the polar axis, resulting in the alignment of the cells between the two poles.\r\n\r\nThere is a second distinction between the types of cleavage in protostomes and deuterostomes. In addition to spiral cleavage, protostomes also undergo <b>determinate cleavage<\/b>. This means that even at this early stage, the developmental fate of each embryonic cell is already determined. A cell does not have the ability to develop into any cell type. In contrast, deuterostomes undergo <b>indeterminate cleavage<\/b>, in which cells are not yet pre-determined at this early stage to develop into specific cell types. These cells are referred to as undifferentiated cells. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem cells, which have the ability to develop into any cell type until their fate is programmed at a later developmental stage.\r\n<div class=\"textbox key-takeaways\">\r\n<h3>The Evolution of the Coelom<\/h3>\r\nOne of the first steps in the classification of animals is to examine the animal\u2019s body. Studying the body parts tells us not only the roles of the organs in question but also how the species may have evolved. One such structure that is used in classification of animals is the coelom. A coelom is a body cavity that forms during early embryonic development. The coelom allows for compartmentalization of the body parts, so that different organ systems can evolve and nutrient transport is possible. Additionally, because the coelom is a fluid-filled cavity, it protects the organs from shock and compression. Simple animals, such as worms and jellyfish, do not have a coelom. All vertebrates have a coelom that helped them evolve complex organ systems.\r\n\r\nAnimals that do not have a coelom are called acoelomates. Flatworms and tapeworms are examples of acoelomates. They rely on passive diffusion for nutrient transport across their body. Additionally, the internal organs of acoelomates are not protected from crushing.\r\n\r\nAnimals that have a true coelom are called eucoelomates; all vertebrates are eucoelomates. The coelom evolves from the mesoderm during embryogenesis. The abdominal cavity contains the stomach, liver, gall bladder, and other digestive organs. Another category of invertebrates animals based on body cavity is pseudocoelomates. These animals have a pseudo-cavity that is not completely lined by mesoderm. Examples include nematode parasites and small worms. These animals are thought to have evolved from coelomates and may have lost their ability to form a coelom through genetic mutations. Thus, this step in early embryogenesis\u2014the formation of the coelom\u2014has had a large evolutionary impact on the various species of the animal kingdom.\r\n\r\n<\/div>\r\n<h2>Hox Genes<\/h2>\r\nSince the early nineteenth\u00a0century, scientists have observed that many animals, from the very simple to the complex, shared similar embryonic morphology and development. Surprisingly, a human embryo and a frog embryo, at a certain stage of embryonic development, look remarkably alike. For a long time, scientists did not understand why so many animal species looked similar during embryonic development but were very different as adults. They wondered what dictated the developmental direction that a fly, mouse, frog, or human embryo would take.\r\n\r\nNear the end of the twentieth\u00a0century, a particular class of genes was discovered that had this very job. These genes that determine animal structure are called \u201chomeotic genes,\u201d and they contain DNA sequences called homeoboxes. The animal genes containing homeobox sequences are specifically referred to as <strong><em>Hox<\/em> genes<\/strong>. This family of genes is responsible for determining the general body plan, such as the number of body segments of an animal, the number and placement of appendages, and animal head-tail directionality. The first <em>Hox<\/em> genes to be sequenced were those from the fruit fly (<em>Drosophila melanogaster<\/em>). A single <em>Hox<\/em> mutation in the fruit fly can result in an extra pair of wings or even appendages growing from the \u201cwrong\u201d body part.\r\n\r\n[caption id=\"attachment_1677\" align=\"alignright\" width=\"449\"]<img class=\"wp-image-1677\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25210840\/Figure_27_01_04.png\" alt=\"This illustration shows the four clusters of Hox genes found in vertebrates: Hox-A, Hox-B, Hox-C, and Hox-D. There are 13 Hox genes, but not all of them are found in each cluster. In both mice and humans, genes 1\u20134 regulate the development of the head. Genes 5 and 6 regulate the development of the neck. Genes 7 and 8 regulate the development of the torso, and genes 9\u201313 regulate the development of the arms and legs.\" width=\"449\" height=\"346\" \/> Figure\u00a06. Shown here is the homology between <em>Hox<\/em> genes in mice and humans. Note how <em>Hox<\/em> gene expression, as indicated with orange, pink, blue and green shading, occurs in the same body segments in both the mouse and the human.[\/caption]\r\n\r\nWhile there are a great many genes that play roles in the morphological development of an animal, what makes <em>Hox <\/em>genes so powerful is that they serve as master control genes that can turn on or off large numbers of other genes. <em>Hox <\/em>genes do this by coding transcription factors that control the expression of numerous other genes. <em>Hox<\/em> genes are homologous in the animal kingdom, that is, the genetic sequences of <em>Hox<\/em> genes and their positions on chromosomes are remarkably similar across most animals because of their presence in a common ancestor, from worms to flies, mice, and humans (Figure\u00a06).\r\n\r\n<em> Hox<\/em> genes are highly conserved genes encoding transcription factors that determine the course of embryonic development in animals. In vertebrates, the genes have been duplicated into four clusters: <em>Hox-A<\/em>, <em>Hox-B<\/em>, <em>Hox-C<\/em>, and <em>Hox-D<\/em>. Genes within these clusters are expressed in certain body segments at certain stages of development.\r\n\r\nOne of the contributions to increased animal body complexity is that <em>Hox<\/em> genes have undergone at least two duplication events during animal evolution, with the additional genes allowing for more complex body types to evolve.\r\n<div class=\"textbox exercises\">\r\n<h3>Practice\u00a0Question<\/h3>\r\nIf a <em>Hox 13<\/em> gene in a mouse was replaced with a <em>Hox 1<\/em> gene, how might this alter animal development?\r\n\r\n[practice-area rows=\"2\"][\/practice-area]\r\n[reveal-answer q=\"319959\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"319959\"]The animal might develop two heads and no tail.[\/hidden-answer]\r\n\r\n<\/div>\r\n<h2><strong>Check Your Understanding<\/strong><\/h2>\r\nAnswer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does\u00a0<strong>not<\/strong>\u00a0count toward your grade in the class, and you can retake it an unlimited number of times.\r\n\r\nUse this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.\r\n\r\nhttps:\/\/assessments.lumenlearning.com\/assessments\/4972","rendered":"<h2>Discuss methods and features of animal reproduction<\/h2>\n<p>Most animals are diploid organisms, meaning that their body (somatic) cells are diploid and haploid reproductive (gamete) cells are produced through meiosis. Some exceptions exist: for example, in bees, wasps, and ants, the male is haploid because it develops from unfertilized eggs. Most animals undergo sexual reproduction. This fact distinguishes animals from fungi, protists, and bacteria, where asexual reproduction is common or exclusive. However, a few groups, such as cnidarians, flatworm, and roundworms, undergo asexual reproduction, although nearly all of those animals also have a sexual phase to their life cycle.<\/p>\n<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<ul>\n<li>Explain the processes of animal reproduction and embryonic development<\/li>\n<li>Compare and contrast the embryonic development of protostomes and deuterostomes<\/li>\n<li>Describe the roles that Hox genes play in development<\/li>\n<\/ul>\n<\/div>\n<h2>Animal Reproduction and Development<\/h2>\n<p>During sexual reproduction, the haploid gametes of the male and female individuals of a species combine in a process called fertilization. Typically, the small, motile male sperm fertilizes the much larger, sessile female egg. This process produces a diploid fertilized egg called a zygote.<\/p>\n<p>Some animal species\u2014including sea stars and sea anemones, as well as some insects, reptiles, and fish\u2014are capable of asexual reproduction. The most common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation, where part of a parent individual can separate and grow into a new individual. In contrast, a form of asexual reproduction found in certain insects and vertebrates is called parthenogenesis (or \u201cvirgin beginning\u201d), where unfertilized eggs can develop into new male offspring. This type of parthenogenesis is called haplodiploidy. These types of asexual reproduction produce genetically identical offspring, which is disadvantageous from the perspective of evolutionary adaptability because of the potential buildup of deleterious mutations. However, for animals that are limited in their capacity to attract mates, asexual reproduction can ensure genetic propagation.<\/p>\n<div id=\"attachment_1675\" style=\"width: 409px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1675\" class=\"wp-image-1675\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25210444\/Figure_27_01_02-1024x862.jpg\" alt=\"Illustration A shows the egg, nymph and adult stages of a grasshopper. The nymph stages are similar in appearance to the adult stage, but smaller. Illustration B shows the egg, larvae, pupa and adult stages of a butterfly. The pupa is a cocoon the butterfly makes when transforming from the larval to adult stages. The winged adult butterfly looks nothing like the caterpillar larva.\" width=\"399\" height=\"336\" \/><\/p>\n<p id=\"caption-attachment-1675\" class=\"wp-caption-text\">Figure\u00a01. (a) The grasshopper undergoes incomplete metamorphosis. (b) The butterfly undergoes complete metamorphosis. (credit: S.E. Snodgrass, USDA)<\/p>\n<\/div>\n<p>After fertilization, a series of developmental stages occur during which primary germ layers are established and reorganize to form an embryo. During this process, animal tissues begin to specialize and organize into organs and organ systems, determining their future morphology and physiology. Some animals, such as grasshoppers, undergo incomplete metamorphosis, in which the young resemble the adult. Other animals, such as some insects, undergo complete metamorphosis where individuals enter one or more larval stages that may in differ in structure and function from the adult (Figure\u00a01). For the latter, the young and the adult may have different diets, limiting competition for food between them. Regardless of whether a species undergoes complete or incomplete metamorphosis, the series of developmental stages of the embryo remains largely the same for most members of the animal kingdom.<\/p>\n<p>The process of animal development begins with the <strong>cleavage<\/strong>, or series of mitotic cell divisions, of the zygote (Figure\u00a02). Three cell divisions transform the single-celled zygote into an eight-celled structure. After further cell division and rearrangement of existing cells, a 6\u201332-celled hollow structure called a <strong>blastula<\/strong> is formed. Next, the blastula undergoes further cell division and cellular rearrangement during a process called gastrulation. This leads to the formation of the next developmental stage, the <strong>gastrula<\/strong>, in which the future digestive cavity is formed. Different cell layers (called <strong>germ layers<\/strong>) are formed during gastrulation. These germ layers are programmed to develop into certain tissue types, organs, and organ systems during a process called <strong>organogenesis<\/strong>.<\/p>\n<div id=\"attachment_1676\" style=\"width: 810px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1676\" class=\"size-full wp-image-1676\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25210530\/Figure_27_01_03.jpg\" alt=\"The left part of the illustration shows a single-celled zygote. The initial cleavage, or cell division, results in a ball of cells, called the eight-cell stage. The cells do not grow during cleavage, so the eight-cell stage ball is about the same diameter as the zygote. Further cleavage results in a hollow ball of cells called a blastula. Upon gastrulation, part of the ball of cells invaginates, forming a cavity called a blastopore.\" width=\"800\" height=\"268\" \/><\/p>\n<p id=\"caption-attachment-1676\" class=\"wp-caption-text\">Figure\u00a02. During embryonic development, the zygote undergoes a series of mitotic cell divisions, or cleavages, to form an eight-cell stage, then a hollow blastula. During a process called gastrulation, the blastula folds inward to form a cavity in the gastrula.<\/p>\n<\/div>\n<div class=\"textbox\">\n<p>Watch the following video to see how human embryonic development (after the blastula and gastrula stages of development) reflects evolution:<\/p>\n<p><script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1593769\/plugins\/11085.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script><br \/>\n<iframe loading=\"lazy\" id=\"myytplayer\" src=\"https:\/\/www.youtube.com\/embed\/uAZmLYWEPGk?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe><\/p>\n<\/div>\n<h2>Embryological Development<\/h2>\n<p>Most animal species undergo a separation of tissues into germ layers during embryonic development. Recall that these germ layers are formed during gastrulation, and that they are predetermined to develop into the animal\u2019s specialized tissues and organs. Animals develop either two or three embryonic germs layers (Figure\u00a03). The animals that display radial symmetry develop two germ layers, an inner layer (endoderm) and an outer layer (ectoderm). These animals are called <b>diploblasts<\/b>. Diploblasts have a non-living layer between the endoderm and ectoderm. More complex animals (those with bilateral symmetry) develop three tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). Animals with three tissue layers are called <b>triploblasts<\/b>.<\/p>\n<div id=\"attachment_1692\" style=\"width: 479px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1692\" class=\"size-full wp-image-1692\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25213129\/Figure_27_02_04.png\" alt=\"The left illustration shows the two embryonic germ layers of a diploblast. The inner layer is the endoderm, and the outer layer is the ectoderm. Sandwiched between the endoderm and the ectoderm is a non-living layer. Right illustration shows the three embryonic germ layers of a triploblast. Like the diploblast, the triploblast has an inner endoderm and an outer ectoderm. Sandwiched between these two layers is a living mesoderm.\" width=\"469\" height=\"269\" \/><\/p>\n<p id=\"caption-attachment-1692\" class=\"wp-caption-text\">Figure\u00a03.\u00a0During embryogenesis, diploblasts develop two embryonic germ layers: an ectoderm and an endoderm. Triploblasts develop a third layer\u2014the mesoderm\u2014between the endoderm and ectoderm.<\/p>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Practice Question<\/h3>\n<p>Which of the following statements about diploblasts and triploblasts is false?<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>Animals that display radial symmetry are diploblasts.<\/li>\n<li>Animals that display bilateral symmetry are triploblasts.<\/li>\n<li>The endoderm gives rise to the lining of the digestive tract and the respiratory tract.<\/li>\n<li>The mesoderm gives rise to the central nervous system.<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q815922\">Show Answer<\/span><\/p>\n<div id=\"q815922\" class=\"hidden-answer\" style=\"display: none\">Statement d is false.<\/div>\n<\/div>\n<\/div>\n<p>Each of the three germ layers is programmed to give rise to particular body tissues and organs. The endoderm gives rise to the lining of the digestive tract (including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures. The ectoderm develops into the outer epithelial covering of the body surface, the central nervous system, and a few other structures. The mesoderm is the third germ layer; it forms between the endoderm and ectoderm in triploblasts. This germ layer gives rise to all muscle tissues (including the cardiac tissues and muscles of the intestines), connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen.<\/p>\n<h3>Presence or Absence of a Coelom<\/h3>\n<p>Further subdivision of animals with three germ layers (triploblasts) results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a <b>coelom<\/b>, and those that do not. This epithelial cell-lined coelomic cavity represents a space, usually filled with fluid, which lies between the visceral organs and the body wall. It houses many organs such as the digestive system, kidneys, reproductive organs, and heart, and contains the circulatory system. In some animals, such as mammals, the part of the coelom called the pleural cavity provides space for the lungs to expand during breathing. The evolution of the coelom is associated with many functional advantages. Primarily, the coelom provides cushioning and shock absorption for the major organ systems. Organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility.<\/p>\n<p>Triploblasts that do not develop a coelom are called <b>acoelomates<\/b>, and their mesoderm region is completely filled with tissue, although they do still have a gut cavity. Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called <b>eucoelomates<\/b> (or coelomates) (Figure\u00a04). A true coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom derived partly from mesoderm and partly from endoderm, which is found between the two layers. Although still functional, these are considered false coeloms, and those animals are called <b>pseudocoelomates<\/b>. The phylum Nematoda (roundworms) is an example of a pseudocoelomate. True coelomates can be further characterized based on certain features of their early embryological development.<\/p>\n<div id=\"attachment_3573\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3573\" class=\"size-large wp-image-3573\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/23214523\/Figure_27_02_05abc-1024x680.jpg\" alt=\"Part a shows the body plan of acoelomates, including flatworms. Acoelomates have a central digestive cavity. Outside this digestive cavity are three tissue layers: an inner endoderm, a central mesoderm, and an outer ectoderm. The photo shows a swimming flatworm, which has the appearance of a frilly black and pink ribbon. Part b shows the body plan of eucoelomates, which include annelids, mollusks, arthropods, echinoderms, and chordates. Eucoelomates have the same tissue layers as acoelomates, but a cavity called a coelom exists within the mesoderm. The coelom is divided into two symmetrical parts that are separated by two spokes of mesoderm. The photo shows a swimming annelid known as a bloodworm. The bloodworm has a tubular body that tapers at each end. Numerous appendages radiate from either side. Part c shows the body plan of pseudocoelomates, which include roundworms. Like the acoelomates and eucoelomates, the pseudocoelomates have an endoderm, a mesoderm, and an ectoderm. However, in pseudocoelomates, a pseudocoelum separates the endoderm from the mesoderm. The photo shows a roundworm, or nematode, which has a tubular body.\" width=\"1024\" height=\"680\" \/><\/p>\n<p id=\"caption-attachment-3573\" class=\"wp-caption-text\">Figure\u00a04.\u00a0Triploblasts may be (a) acoelomates, (b) eucoelomates, or (c) pseudocoelomates. Acoelomates have no body cavity. Eucoelomates have a body cavity within the mesoderm, called a coelom, which is lined with mesoderm. Pseudocoelomates also have a body cavity, but it is sandwiched between the endoderm and mesoderm. (credit a: modification of work by Jan Derk; credit b: modification of work by NOAA; credit c: modification of work by USDA, ARS)<\/p>\n<\/div>\n<h3>Embryonic Development of the Mouth<\/h3>\n<div id=\"attachment_1695\" style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1695\" class=\"wp-image-1695\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25213455\/Figure_27_02_06.jpg\" alt=\"The illustration compares the development of protostomes and deuterostomes. In both protostomes and deuterostomes, the gastrula, which resembles a hollow ball of cells, contains an indentation called a blastopore. In protostomes, two circular layers of mesoderm form inside the gastrula, containing the coelom cavity. As the protostome develops, the mesoderm grows and fuses with the gastrula cell layer. The blastopore becomes the mouth, and a second opening forms opposite the mouth, which becomes the anus. In deuterostomes, two groups of gastrula cells in the blastopore grow inward to form the mesoderm. As the deuterostome develops, the mesoderm pinches off and fuses, forming a second body cavity. The body plan of the deuterostome at this stage looks very similar to that of the protostome, but the blastopore becomes the anus, and the second opening becomes the mouth.\" width=\"350\" height=\"380\" \/><\/p>\n<p id=\"caption-attachment-1695\" class=\"wp-caption-text\">Figure\u00a05.\u00a0Eucoelomates can be divided into two groups based on their early embryonic development. In protostomes, part of the mesoderm separates to form the coelom in a process called schizocoely. In deuterostomes, the mesoderm pinches off to form the coelom in a process called enterocoely. It was long believed that the blastopore developed into the mouth in protostomes and into the anus in deuterostomes, but recent evidence challenges this belief.<\/p>\n<\/div>\n<p>Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in their early embryonic development. <b>Protostomes<\/b> include arthropods, mollusks, and annelids. <b>Deuterostomes<\/b> include more complex animals such as chordates but also some simple animals such as echinoderms. These two groups are separated based on which opening of the digestive cavity develops first: mouth or anus. The word protostome comes from the Greek word meaning \u201cmouth first,\u201d and deuterostome originates from the word meaning \u201cmouth second\u201d (in this case, the anus develops first). The mouth or anus develops from a structure called the blastopore (Figure\u00a05).<\/p>\n<p>The <b>blastopore<\/b> is the indentation formed during the initial stages of gastrulation. In later stages, a second opening forms, and these two openings will eventually give rise to the mouth and anus (Figure\u00a05). It has long been believed that the blastopore develops into the mouth of protostomes, with the second opening developing into the anus; the opposite is true for deuterostomes. Recent evidence has challenged this view of the development of the blastopore of protostomes, however, and the theory remains under debate.<\/p>\n<p>Another distinction between protostomes and deuterostomes is the method of coelom formation, beginning from the gastrula stage. The coelom of most protostomes is formed through a process called <b>schizocoely<\/b>, meaning that during development, a solid mass of the mesoderm splits apart and forms the hollow opening of the coelom. Deuterostomes differ in that their coelom forms through a process called <b>enterocoely<\/b>. Here, the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse to form the mesoderm, which then gives rise to the coelom.<\/p>\n<p>The earliest distinction between protostomes and deuterostomes is the type of cleavage undergone by the zygote. Protostomes undergo <b>spiral cleavage<\/b>, meaning that the cells of one pole of the embryo are rotated, and thus misaligned, with respect to the cells of the opposite pole. This is due to the oblique angle of the cleavage. Deuterostomes undergo <b>radial cleavage<\/b>, where the cleavage axes are either parallel or perpendicular to the polar axis, resulting in the alignment of the cells between the two poles.<\/p>\n<p>There is a second distinction between the types of cleavage in protostomes and deuterostomes. In addition to spiral cleavage, protostomes also undergo <b>determinate cleavage<\/b>. This means that even at this early stage, the developmental fate of each embryonic cell is already determined. A cell does not have the ability to develop into any cell type. In contrast, deuterostomes undergo <b>indeterminate cleavage<\/b>, in which cells are not yet pre-determined at this early stage to develop into specific cell types. These cells are referred to as undifferentiated cells. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem cells, which have the ability to develop into any cell type until their fate is programmed at a later developmental stage.<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>The Evolution of the Coelom<\/h3>\n<p>One of the first steps in the classification of animals is to examine the animal\u2019s body. Studying the body parts tells us not only the roles of the organs in question but also how the species may have evolved. One such structure that is used in classification of animals is the coelom. A coelom is a body cavity that forms during early embryonic development. The coelom allows for compartmentalization of the body parts, so that different organ systems can evolve and nutrient transport is possible. Additionally, because the coelom is a fluid-filled cavity, it protects the organs from shock and compression. Simple animals, such as worms and jellyfish, do not have a coelom. All vertebrates have a coelom that helped them evolve complex organ systems.<\/p>\n<p>Animals that do not have a coelom are called acoelomates. Flatworms and tapeworms are examples of acoelomates. They rely on passive diffusion for nutrient transport across their body. Additionally, the internal organs of acoelomates are not protected from crushing.<\/p>\n<p>Animals that have a true coelom are called eucoelomates; all vertebrates are eucoelomates. The coelom evolves from the mesoderm during embryogenesis. The abdominal cavity contains the stomach, liver, gall bladder, and other digestive organs. Another category of invertebrates animals based on body cavity is pseudocoelomates. These animals have a pseudo-cavity that is not completely lined by mesoderm. Examples include nematode parasites and small worms. These animals are thought to have evolved from coelomates and may have lost their ability to form a coelom through genetic mutations. Thus, this step in early embryogenesis\u2014the formation of the coelom\u2014has had a large evolutionary impact on the various species of the animal kingdom.<\/p>\n<\/div>\n<h2>Hox Genes<\/h2>\n<p>Since the early nineteenth\u00a0century, scientists have observed that many animals, from the very simple to the complex, shared similar embryonic morphology and development. Surprisingly, a human embryo and a frog embryo, at a certain stage of embryonic development, look remarkably alike. For a long time, scientists did not understand why so many animal species looked similar during embryonic development but were very different as adults. They wondered what dictated the developmental direction that a fly, mouse, frog, or human embryo would take.<\/p>\n<p>Near the end of the twentieth\u00a0century, a particular class of genes was discovered that had this very job. These genes that determine animal structure are called \u201chomeotic genes,\u201d and they contain DNA sequences called homeoboxes. The animal genes containing homeobox sequences are specifically referred to as <strong><em>Hox<\/em> genes<\/strong>. This family of genes is responsible for determining the general body plan, such as the number of body segments of an animal, the number and placement of appendages, and animal head-tail directionality. The first <em>Hox<\/em> genes to be sequenced were those from the fruit fly (<em>Drosophila melanogaster<\/em>). A single <em>Hox<\/em> mutation in the fruit fly can result in an extra pair of wings or even appendages growing from the \u201cwrong\u201d body part.<\/p>\n<div id=\"attachment_1677\" style=\"width: 459px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1677\" class=\"wp-image-1677\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25210840\/Figure_27_01_04.png\" alt=\"This illustration shows the four clusters of Hox genes found in vertebrates: Hox-A, Hox-B, Hox-C, and Hox-D. There are 13 Hox genes, but not all of them are found in each cluster. In both mice and humans, genes 1\u20134 regulate the development of the head. Genes 5 and 6 regulate the development of the neck. Genes 7 and 8 regulate the development of the torso, and genes 9\u201313 regulate the development of the arms and legs.\" width=\"449\" height=\"346\" \/><\/p>\n<p id=\"caption-attachment-1677\" class=\"wp-caption-text\">Figure\u00a06. Shown here is the homology between <em>Hox<\/em> genes in mice and humans. Note how <em>Hox<\/em> gene expression, as indicated with orange, pink, blue and green shading, occurs in the same body segments in both the mouse and the human.<\/p>\n<\/div>\n<p>While there are a great many genes that play roles in the morphological development of an animal, what makes <em>Hox <\/em>genes so powerful is that they serve as master control genes that can turn on or off large numbers of other genes. <em>Hox <\/em>genes do this by coding transcription factors that control the expression of numerous other genes. <em>Hox<\/em> genes are homologous in the animal kingdom, that is, the genetic sequences of <em>Hox<\/em> genes and their positions on chromosomes are remarkably similar across most animals because of their presence in a common ancestor, from worms to flies, mice, and humans (Figure\u00a06).<\/p>\n<p><em> Hox<\/em> genes are highly conserved genes encoding transcription factors that determine the course of embryonic development in animals. In vertebrates, the genes have been duplicated into four clusters: <em>Hox-A<\/em>, <em>Hox-B<\/em>, <em>Hox-C<\/em>, and <em>Hox-D<\/em>. Genes within these clusters are expressed in certain body segments at certain stages of development.<\/p>\n<p>One of the contributions to increased animal body complexity is that <em>Hox<\/em> genes have undergone at least two duplication events during animal evolution, with the additional genes allowing for more complex body types to evolve.<\/p>\n<div class=\"textbox exercises\">\n<h3>Practice\u00a0Question<\/h3>\n<p>If a <em>Hox 13<\/em> gene in a mouse was replaced with a <em>Hox 1<\/em> gene, how might this alter animal development?<\/p>\n<p><textarea aria-label=\"Your Answer\" rows=\"2\"><\/textarea><\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q319959\">Show Answer<\/span><\/p>\n<div id=\"q319959\" class=\"hidden-answer\" style=\"display: none\">The animal might develop two heads and no tail.<\/div>\n<\/div>\n<\/div>\n<h2><strong>Check Your Understanding<\/strong><\/h2>\n<p>Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does\u00a0<strong>not<\/strong>\u00a0count toward your grade in the class, and you can retake it an unlimited number of times.<\/p>\n<p>Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.<\/p>\n<p>\t<iframe id=\"lumen_assessment_4972\" class=\"resizable\" src=\"https:\/\/assessments.lumenlearning.com\/assessments\/load?assessment_id=4972&#38;embed=1&#38;external_user_id=&#38;external_context_id=&#38;iframe_resize_id=lumen_assessment_4972\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:400px;\"><br \/>\n\t<\/iframe><\/p>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-1867\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Original<\/div><ul class=\"citation-list\"><li>Introduction to Animal Reproduction. <strong>Authored by<\/strong>: Shelli Carter and Lumen Learning. <strong>Provided by<\/strong>: Lumen Learning. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em><\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">All rights reserved content<\/div><ul class=\"citation-list\"><li>What Can Embryos Tell Us About Evolution?. <strong>Authored by<\/strong>: The Science Channel . <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/youtu.be\/uAZmLYWEPGk\">https:\/\/youtu.be\/uAZmLYWEPGk<\/a>. <strong>License<\/strong>: <em>All Rights Reserved<\/em>. <strong>License Terms<\/strong>: Standard YouTube License<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":8,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\"},{\"type\":\"original\",\"description\":\"Introduction to Animal Reproduction\",\"author\":\"Shelli Carter and Lumen Learning\",\"organization\":\"Lumen Learning\",\"url\":\"\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"\"},{\"type\":\"copyrighted_video\",\"description\":\"What Can Embryos Tell Us About Evolution?\",\"author\":\"The Science Channel \",\"organization\":\"\",\"url\":\"https:\/\/youtu.be\/uAZmLYWEPGk\",\"project\":\"\",\"license\":\"arr\",\"license_terms\":\"Standard YouTube License\"}]","CANDELA_OUTCOMES_GUID":"bb551b62-72e2-4a37-9868-a4b7e0a7a87a, 2d5c1b17-faad-4c6e-9838-753ca8853c84, 3b591d78-8763-4f81-b865-92f8607321d6, 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