During gastrulation, diploblastic animals form two main layers: 1. Ectoderm external layer 2. Endoderm internal layer. In triploblastic animals, three germ layers are present: 1. Mesoderm middle layer 3. Diploblastic animals do not have a mesodermal layer. Triploblastic animals have a mesodermal layer that forms notochord, true organs, bones, muscles, connective tissue, and circulatory system. A non-living layer named mesoglea is present between ectoderm and endoderm. Mesoglea helps in protecting the gut lining and body.
Triploblastic animals do not have mesoglea. The endoderm of diploblastic animals has true tissues and intestines. The endoderm of triploblastic animals has lungs, liver, stomach, colon, and urinary bladder. The ectoderm of diploblastic animals form nephridia, nervous tissue, and epidermis.
The ectoderm of triploblastic animals forms the brain, spinal cord, blood, and lens of the eye. These animals are symmetrical radially. These animals are symmetrical bilaterally. They are complex animals as compared to diploblastic. Removal of a blastomere from an embryo with determinate cleavage can result in missing structures, and embryos that fail to develop. In contrast, deuterostomes undergo indeterminate cleavage , in which cells are not yet fully committed at this early stage to develop into specific cell types.
Removal of individual blastomeres from these embryos does not result in the loss of embryonic structures. In fact, twins clones can be produced as a result from blastomeres that have been separated from the original mass of blastomere cells. Unlike protostomes, however, if some blastomeres are damaged during embryogenesis, adjacent cells are able to compensate for the missing cells, and the embryo is not damaged.
These cells are referred to as undetermined 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. One structure that is used in classification of animals is the body cavity or coelom. The body cavity develops within the mesoderm, so only triploblastic animals can have body cavities.
Therefore body cavities are found only within the Bilateria. In other animal clades, the gut is either close to the body wall or separated from it by a jelly-like material. The body cavity is important for two reasons. Fluid within the body cavity protects the organs from shock and compression. In addition, since in triploblastic embryos, most muscle, connective tissue, and blood vessels develop from mesoderm, these tissues developing within the lining of the body cavity can reinforce the gut and body wall, aid in motility, and efficiently circulate nutrients.
To recap what we have discussed above, animals that do not have a coelom are called acoelomates. The major acoelomate group in the Bilateria is the flatworms, including both free-living and parasitic forms such as tapeworms. In these animals, mesenchyme fills the space between the gut and the body wall.
Although two layers of muscle are found just under the epidermis, there is no muscle or other mesodermal tissue around the gut. Flatworms rely on passive diffusion for nutrient transport across their body. In pseudocoelomates , there is a body cavity between the gut and the body wall, but only the body wall has mesodermal tissue. In these animals, the mesoderm forms, but does not develop cavities within it. Major pseudocoelomate phyla are the rotifers and nematodes. Animals that have a true coelom are called eucoelomates ; all vertebrates, as well as molluscs, annelids, arthropods, and echinoderms, are eucoelomates.
The coelom develops within the mesoderm during embryogenesis. Of the major bilaterian phyla, the molluscs, annelids, and arthropods are schizocoels , in which the mesoderm splits to form the body cavity, while the echinoderms and chordates are enterocoels , in which the mesoderm forms as two or more buds off of the gut. These buds separate from the gut and coalesce to form the body cavity. In the vertebrates, mammals have a subdivided body cavity, with the thoracic cavity separated from the abdominal cavity.
The pseudocoelomates may have had eucoelomate ancestors and may have lost their ability to form a complete coelom through genetic mutations. Mangold harvested presumptive ectoderm from the dorsal lip, a tissue that organizes the gastrula stage, of an embryonic newt and transplanted this tissue to a different germ layer of the gastrula of a second species of newt. The transplanted ectoderm responded to the local environment on the developing host newt, and induced the formation of an extra head, nervous system structure, or extra body.
That experiment demonstrated that the fates of germ layer cells are not entirely predetermined at the start of development. In the fifteen years following Mangold's work, embryologists continued to explore the potential for the three germ layers to differentiate along different routes and they produced evidence that further weakened the germ layer theory.
He employed transplantation, recombination, and fate mapping experiments to investigate the capacity of the germ layers to transform into tissues atypical of normal differentiation.
Throughout the remainder of the twentieth century, researchers continued to accumulate evidence that invalidated the theory that germ layers are pre-defined or highly-fated tissues. In the early s Robert Briggs, at Indiana University in Bloomington, Indiana, and Thomas King, at the Institute for Cancer Research in Philadelphia, Pennsylvania, transplanted nuclei from the presumptive endoderm of the northern leopard frog , Rana pipiens , into eggs from which they had removed the nuclei.
Briggs and King tracked the development of these transplanted nuclei to explore the timing of cell differentiation , and with those experiments they laid the foundation for future research in cloning. In the late s Pieter D. Nieuwkoop, at the Hubrecht Laboratory in the Royal Netherlands Academy of Arts and Science, in Utrecht, Holland, discovered that endoderm induces adjacent ectoderm to form mesoderm.
In the s scientists shifted their focus towards identifying the genes that induce structural differentiation of the germ layers. Researchers in the early twenty-first century investigated the regulatory networks through which individual genes interact to cause germ layer differentiation. Keywords: Germ layer theory , mesoderm , endoderm , ectoderm. Germ Layers A germ layer is a group of cells in an embryo that interact with each other as the embryo develops and contribute to the formation of all organs and tissues.
Sources von Baer, Karl Ernst. Observation and Reflection]. Briggs, Robert, and Thomas King. Darwin, Charles. London: Murray, Developmental Biology. Massachusetts: Sinauer, Haeckel, Ernst. In Jenaische Zeitschrift fur Naturwissenschaft , 8 : 1— Hall, Brian Keith. Some exceptions exist: For example, in bees, wasps, and ants, the male is haploid because it develops from an unfertilized egg.
Most animals undergo sexual reproduction, while many also have mechanisms of asexual reproduction. Almost all animal species are capable of reproducing sexually; for many, this is the only mode of reproduction possible. This distinguishes animals from fungi, protists, and bacteria, where asexual reproduction is common or exclusive.
During sexual reproduction, the male and female gametes of a species combine in a process called fertilization. Typically, the small, motile male sperm travels to the much larger, sessile female egg. Sperm form is diverse and includes cells with flagella or amoeboid cells to facilitate motility.
Fertilization and fusion of the gamete nuclei produce a zygote. Fertilization may be internal, especially in land animals, or external, as is common in many aquatic species. After fertilization, a developmental sequence ensues as cells divide and differentiate. Many of the events in development are shared in groups of related animal species, and these events are one of the main ways scientists classify high-level groups of animals.
During development, animal cells specialize and form tissues, determining their future morphology and physiology. In many animals, such as mammals, the young resemble the adult. Other animals, such as some insects and amphibians, undergo complete metamorphosis in which individuals enter one or more larval stages.
For these animals, the young and the adult have different diets and sometimes habitats. In other species, a process of incomplete metamorphosis occurs in which the young somewhat resemble the adults and go through a series of stages separated by molts shedding of the skin until they reach the final adult form.
Asexual reproduction, unlike sexual reproduction, produces offspring genetically identical to each other and to the parent. A number of animal species—especially those without backbones, but even some fish, amphibians, and reptiles—are capable of asexual reproduction. Asexual reproduction, except for occasional identical twinning, is absent in birds and mammals. The most common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation, in which part of a parent individual can separate and grow into a new individual.
Animals are classified according to morphological and developmental characteristics, such as a body plan. With the exception of sponges, the animal body plan is symmetrical. This means that their distribution of body parts is balanced along an axis. Additional characteristics that contribute to animal classification include the number of tissue layers formed during development, the presence or absence of an internal body cavity, and other features of embryological development.
Animals may be asymmetrical, radial, or bilateral in form [Figure 3]. Asymmetrical animals are animals with no pattern or symmetry; an example of an asymmetrical animal is a sponge [Figure 3] a. An organism with radial symmetry [Figure 3] b has a longitudinal up-and-down orientation: Any plane cut along this up—down axis produces roughly mirror-image halves.
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