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| Gastrulation |
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| Gastrulation is an important dynamic process in the development of the zygote which involves movement of cell masses from the surface of the blastula to their definitive positions in the embryo and their three primary germinal layers i.e., ectoderm, endoderm and mesoderm. These movements are called as morphogenetic movements. |
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| The enlargement of the blastodermic vesicle is followed by the detachment of some cells from the inner cell mass. The detached cells are pushed into the blastocoel to form the embryonal endoderm or hypoblast. These cells rapidly increase in number to constitute the second complete layer inside the original outer layer of the blastodermic vesicle. |
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| Stages in the Development of Endoderm |
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| A section of the embryo at this stage shows a tube enclosed within a tube. This inner tube is bounded by the endoderm. It forms the primitive gut. At a later stage, the primitive gut differentiates into 2 parts. The parts of the gut in the embryonal part forms the gut tract. The other portion as a distal sac forms the yolk sac that communicates with the gut of the embryo. |
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| After the formation of the endoderm, the remaining mass of cell of the inner cell mass gets consolidated by the regular arrangement of cells. This mass is called the embryonic disc. |
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| After the completion of the endoderm as a cellular layer, cells from the caudal margin of the embryonic disc, start proliferating at an increased rate. This results in an increase in the thickening of the disc at that end. These cells then detach themselves and spread next to the endodermis to form the mesoderm. |
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| After the formation of the mesoderm the remaining cells of the embryonic disc arrange themselves outer to the mesoderm to form the ectoderm. |
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| Significance of Gastrulation |
 Three primary germinal layers are formed. |
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 It will lead to differentiation and morphogenesis. |
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| Neurulation |
| Gastrulation is followed by neurulation, during which the primordium of nervous system, the neural plate is laid down. |
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| Organogenesis |
| Neuruletion is followed by organogenesis. During this stage, various organs of the foetus are established and they become functional. |
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| Stages in the Formation of Mesoderm in a Mammalian Embryo |
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| The 3 germ layers give rise to definite organs and organ systems. The different organs formed from the different layers are |
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| Germ Layer - Structures formed |
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| Ectoderm - Epidermis, lens retina, internal ear, CNS, middle and posterior pituitary and pineal gland |
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| Mesoderm - Muscles, connective tissue, dermis, circulatory system, kidney, ureters, gonads. |
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| Endoderm - Midgut, bladder, lungs, liver, pancreas, thyroid, thymus, anterior pituitary. |
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| These membranes are formed outside the embryo from the trophoblast and perform specific functions. |
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| Embryonic Membranes and Placental Villi |
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| It develops above the embryo. It consists of the trophoblast inside and mesoderm outside. Between the amnion and the embryo, there is the amniotic cavity filled with the amniotic fluid secreted by the embryo and the amnion. |
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| The amnion and the fluid filled amniotic cavity enlarge and nearly surround the embryo. The embryo is suspended in the amniotic cavity by a thick umbilical cord, which is attached to the midgut region of the embryo. The main blood vessels from the placenta reach the foetus through the umbilical cord. |
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 The amnion protects the embryo from injury. |
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 The amniotic fluid absorbs shock and prevents desiccation of the embryo. |
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| While the amniotic cavity is being formed, another cavity develops. This arises as an out growth from the hindgut which pushes its way into extra embryonic coelom. It is known as allantois. |
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| It stores the nitrogenous wastes of the embryo, so it acts as an extra embryonic kidney. In eutherian mammals it takes part in placenta formation. |
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| It is the outermost layer and completely surrounds the embryo. It is composed of trophoblast outside and mesoderm on the inner side. |
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| It fuses with the lining of allantion to form allanto - chorion. |
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| It acts as an extra embryonic lung and helps in exchange of gases. It also plays an important part in embryonic respiration, excretion and nutrition by developing into a placenta. |
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| It is well developed in reptiles, birds and prototherians. It is mainly digestive in function so it acts as an extra embryonic gut. In human beings it is vestigial. |
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